Plug N Play Friends

"The Gamemaster: Guys, please! I want you to roleplay this. Remember you've never met this guy before, the last guys you met tried to kill you, and you're standing in the ruins of an evil, cursed castle. Just act appropriately. Magellan: Hello, I'm Magellan, a traveling mage. I notice your group has no wizard. Rogar: You seem trustworthy. Would you care to join us in our noble quest? Magellan: Yes. Yes I would."

- The Gamers

When a new character permanently joins an ensemble cast, it takes an astonishingly short time for her to get to the same level of rapport with the other characters as they already had with each other (and with the character she's replacing). Expect one Fish Out of Water episode, and then she'll be flirting, sharing intimate secrets, in on all the jokes.

In an action series or a video game, she'll be willing to die for everyone else, and vice versa, with a passion nobody ever shows for the one-episode Millstone character. Of course, her outsider status could be resurrected for a one-episode conflict later, or she could turn out to be The Mole. (if so, and she regrets her actions, she will be Easily Forgiven)

Especially jarring in Heel Face Turn scenarios.

Anime and Manga

 * In Black Lagoon, Rock integrates rather quickly with the crew of the Lagoon, insofar he goes from being their hostage to being, at least superficially, 'one of them' in the space of about a day (although to be fair, that was some day). Rock himself Lampshades this at one point and starts to wonder if he's developed Stockholm Syndrome because of how calm he is over the degree of familiarity he gets with them so quickly. Revy is the exception: it takes her about six episodes to properly accept him once he has actually joined them.
 * to be fair, Rock did save all of their asses by devising the most balls-to-the-wall insane method of killing an attack helicopter ever, so its not as if he just showed up with a gift basket and asked if he could join.
 * One of many reasons that Shirayuki Berii of Tokyo Mew Mew a la mode is marked as a Canon Sue. The Nakama is generally inclusive and friendly, but Berii makes them practically worship her before they even speak to her.
 * This is a frequent occurrence with new members of the Strawhat Pirates in One Piece, although Luffy is the kind of guy who'd be quick to be friendly with anyone he meets unless they do something to get on his bad side. One particular weird instance is how Sanji is completely sure Nami would never betray them during the Arlong Park Arc... even though he just joined the crew in the previous arc, which Nami was mostly absent for, and thus barely had any contact with her. The only noticeable exception is Robin in how she never refers to any of her fellow crew mates by name until the end of the Enies Lobby Arc.
 * How is it weird that Sanji was acting completely in-character? His two defining character traits are being an expert chef and falling in love with every pretty girl he lays eyes on. Considering how he acts the rest of the time, him being suspicious of Nami would practically be character derailment... (remember that he was very quick to accept Robin as a member of the crew, even knowing she had just defected from Baroque Works, just because)

Literature

 * David from Animorphs could almost be considered a deconstruction of this; the Animorphs (except Marco and Ax) expect a relative stranger to join their team, cooperate with them, and go along with their tactics. However, the Animorphs play their hand poorly, and David is a bit too selfish for their tastes. It doesn't end well.

Live Action TV

 * Oliver in Coupling. Goes on a blind date with one of the main characters and it doesn't work out. In subsequent episodes he gets involved in the six-way phone call, and Jane later invites him to Sally's dinner party and Susan's antenatal class. By his fifth episode he's having regular drinks with Steve and Patrick, in Jeff's old seat.
 * This trope is also employed in the second episode, when Patrick befriends then-strangers Steve and Jeff, although Jane takes until the third episode to make friends with Susan. Played straighter with Jane and Sally, though.
 * Rachel in Friends, although she does have some prior connections.
 * Stargate SG-1's introduction of Teal'c, in which he betrays his god in a Heel Face Turn, makes it seem odd that O'Neill trusts him so quickly, to the point of sternly insisting on his inclusion into SG-1. Jonas Quinn averted it, however.
 * Averted in Stargate Atlantis when Ronon was added to the cast as a replacement. 2 and a half seasons later and the majority of the main characters are still somewhat scared of him.
 * Eh, not really. It takes Ronon a few episodes to become comfortable with the team and start forming deep relationships with them (a dead girlfriend and several years of being unable to stay in any one place for very long without endangering lives will do that to you.) He definitely takes a little longer to mesh with the team than most examples on this page, but they're all pretty tight fairly quickly.
 * Common in Power Rangers, each time one of the rangers is replaced.
 * Each time the rangers meet someone who's not obviously a Monster of the Week.
 * All the Maquis characters in Star Trek: Voyager. By half way through the first season (once they get rid of Seska), none of the Starfleet personnel seem to care they're working with terrorists, and none of the Maquis seem to remember that they believe Starfleet and the Federation sold them down the river. Granted, they've got more important things to worry about, but still, you'd think it rated an occasional mention.
 * It got Lampshaded when they finally established contact with Star Fleet, when it turned out that their superiors had been extremely concerned about it, while they had mostly just forgot about it.
 * And it was referenced in the episode Worst Case Scenario, which centered around a training program based on the idea of a Maquis rebellion.
 * There was another episode when Tuvok had to deal with some discipline problems among low-ranking Maquis. When they complained to Chakotay that they prefer the Maquis way, he punches one of them in the face and tells them to go give the Starfleet way another try. By the end of the episode, they all trust Tuvok and he trusts them.
 * For that matter, the crew accepts Neelix, Kes, and even Seven remarkably quickly, considering two of them were strangers from previously unknown alien races and the third was an enemy combatant. Neelix's first act was actually to use their help under false pretenses to rescue Kes.
 * One episode after B.J. was introduced in M*A*S*H, he was inexplicably Hawkeye's best bud and the incoming Colonel Potter, looking over their records, berated them for how much goofing off they did together. This after B.J. had been introduced as a straight arrow in the previous episode. One has to wonder if the episode was written before Wayne Rogers left and they just crossed out "Trapper" and wrote in "B.J." without changing anything else.
 * According to a "behind the scenes" show on the Biography Channel, this is exactly what they did.
 * This happens in season 3 of NCIS after the character of Caitlin Todd is killed off, her replacement is Mossad Officer Ziva David, who joins the team only a couple of episodes after her half-brother (the terrorist Ari Haswari) murdered Caitlin. However, this could be seen as an aversion, since they show the other members of the team being fairly open to her, but commenting on how it is odd for their boss, Agent Gibbs, to do the same. This is explained as Ziva having saved Gibbs' life when Ari came to kill him in his basement (by shooting her half-brother), forming a bond of trust that otherwise would not have occurred nearly as fast.
 * H.G.Wells in the last episodes of season two on Warehouse 13.She joins the team suddenly in one episode, treated as family the next, before she betrays them in the season finale.
 * Averted in Dead Like Me with Daisy. It takes the rest of the reapers quite a while to get used to her.

Professional Wrestling

 * In pro-wrestling, Heels are generally friends with Heels, Faces with Faces. When a Face turns Heel, it is often quite strange, as they become friends with wrestlers who have attempted to do various despicable things to them in the past. On one memorable occasion, Stone Cold Steve Austin turned heel and teamed up with Triple H... who had previously ordered Stone Cold to be run down, in an attempt to end his career or kill him. Often the whole thing comes with copious amounts of Lampshade Hanging, whether immediately (the "I don't like you and you don't like me, but let's team against our common enemies" speech) or down the road as a Continuity Nod (witness the interview with Triple H's team at Survivor Series 2007, where Kane and Jeff Hardy call in their receipts for the various heinous behaviors that HHH has subjected them to over the years when he was a Heel).

Video Games

 * Pick a role-playing game that doesn't involve the player creating the entire party from the start. Odds are, this trope will show up in it at least once.
 * Tales of Symphonia was particularly bad about this. Sheena fights the group twice before joining up and just a few scenes later she's giving advice to Colette (which she takes to heart). Then you have characters like Zelos and Presea who they just meet, but are already talking to as if they are "one of the guys."
 * Though Lloyd does spend about... Ten minutes somewhat miffed over the level of familiarity Zelos presumes with the rest of the party. After which he seems to have resigned himself to it -- Zelos is, to put it mildly, easygoing. Presea never really integrates until her exsphere is brought under control, at which point the party are already well familiar with her.
 * And, uh... this IS Colette we're talking about. She would have taken advice from Sheena to heart if the woman was still actively trying to kill her!
 * Not to mention that, even as Sheena was trying to kill her, Colette was more then happy to try to make friends with her and held her in high regard (see: pre-battle conversations and post-battle Z-skits). The speed at which the rest of the party accepted here tended to be a bit more reasonable, though by the time they reached the Tower of Salvation (either very soon after Sheena joined or quite a time later depending on which route you took), even Raine was willing to trust her. (see: No real questions about her summoning skill and )
 * Fire Emblem. This happens so much, it is a wonder that the hero's group doesn't pick up any spies.  In Fire Emblem, however, in the games where you're merceneries everybody seems to think it perfectly normal to end up killing your old friends (in Gatrie's words) 'because of their poor choice in employers', so maybe it follow that it's normal to befriend the people who were trying to kill you.
 * Played quite straight in Yggdra Union: We'll Never Fight Alone, except the joining enemy was trying to kill you for 26 maps, and it takes less than 1 map to try and risk the male lead's life to save her. Either they're too confident, or too idiotic.
 * On that note, she's probably the only one who actually makes any attempt to prove her trustworthiness. Everyone else is all, "Okay, we trust you already, stop trying so hard." Oh, and don't forget, although he had an excuse.
 * Happens a lot in Exit Fate, as the game has 75 recruitable party members.
 * Petra just joins the army because she doesn't think you respect a woman's strength. Shin is almost literally an incarnation of evil and tried to turn Soth into something similar and you have to fight them, but then Soth joins up to redeem himself and Shin joins because you seem interesting. The vampire Vanrushal takes the cake though -- after you and the exorcist Luther confront and fight him in his evil mansion (after killing dozens of evil minions) he claims that he's weakened down to near-human levels and can't stay here, so he joins your army. Without your approval. Against Luther's loud protests. (Luther joins too, to keep an eye on Vanrushal.) Granted, as Vanrushal points out, he was keeping to himself and it was your party that tried to kill him, and he wasn't even an evil vampire -- Luther's just a (fairly benign) zealot when it comes to vampire hunting.
 * Implied by the way the Relationship Values work in the game -- while some characters are obviously friends with each other from onscreen interaction, other characters instantly gain a relationship bonus with other characters offscreen. Why a cat and a dog are friends with each other, and why two Cool Old Guys happen to be buddies, is completely up to speculation.

Web Comics

 * The Trolls and Kids to each other in Homestuck, considering they've known each other for all of a day on positive terms, although the Trolls harassed them previously. Averted however with some of the Trolls who outright refuse or seem disinterested in getting to know the Kids.
 * This is incidentally the source of a major Base Breaker, in that canonical romances are beginning between some of them, again having known each other for under 24 hours.

Western Animation

 * Terra from Teen Titans meets the main cast in one episode, becomes best buds with them in a minute, then leaves in a fit of rage after (mistakenly) thinking they had betrayed her at the end of that same episode. Later, she comes back with a big smile on her face, offering her unending services to the team. They take her in, they throw her a big old welcome party (with the exception of Raven, who dryly tells Terra to help herself to the fridge and to make sure she cleans up after herself before she inevitably leaves), and then she attempts to murder them all. This is over-simplified; not only is the Plug N Play Friends aspect ginormous here, Terra herself is actually very well developed as a character and the reasons for her actions are not hard to discern.
 * Another episode has Blackfire, Starfire's older sister. Very jarring in that she compliments each character (except Starfire, obviously) and instantly has their friendship, even Raven. By acting buddy-buddy with Cyborg, laughing at Beastboy's jokes, noting Raven's forehead chakra, and flirting with Robin. Then she turns out to be a thief and tries to kill Starfire. Oops.
 * In Blackfire's case, the Titans were probably trying to be extra nice to her because she was Starfire's sister.
 * Averted twice in Avatar: The Last Airbender:
 * Aang welcomes Toph immediately, but he likes everyone. Sokka welcomes her as a powerful ally from the start, but doesn't seem to relate to her as a friend for some time. Katara, on the other hand, oscillates between obvious efforts to get closer to Toph and fighting with her well into the following SEASON.
 * When joins the Gaang, he is also not welcomed immediately. He needs a separate episode with EACH of Aang, Sokka, and Katara to get their acceptance and forgiveness.
 * Sadly, no life-changing field trip with Zuko for Toph.

Other

 * Common in tabletop RPGs when a PC gets killed and the player has to make a replacement character.
 * Done hilariously in the film The Gamers
 * In LARP is even worse, as the "new guy" is remarkable similar to the recently lost one. Possibly because it's the same player with a different costume (and not always). Worth noting that when players try to play it straight and to build a new relation from zero, it staggers the game a lot, and often kills it outright.
 * Brookes in Breakout - going from pretty much nobody to an integral member of the crew within all of about 10 minutes.