Not Right in the Bed

One sign that someone's been possessed, cloned, body-swapped etc. is when they start behaving in a strange way romantically.

This typically means they're far more sexual than usual. Cue Fan Service.

Another variation that crops up when evil clones or evil body doubles appears is that the copy is generally a really bad kisser, giving a clue to their true identity.

See also Showing Off the New Body. Compare Good People Have Good Sex (because being more sexually adventurous than normal is a sign of villainy).

Anime and Manga

 * Mahou Sensei Negima has several versions of this:
 * First, Negi accidentally creates a bunch of magical duplicates of himself. Naturally, they start hitting on his students.
 * Later, during the Kyoto arc, the girls use a bunch of the same magical duplicates to take their places while they visit Konoka's temple complex. When they get back, the duplicates are doing a Striptease.
 * Even later, Haruna uses her artifact to make a "dark" copy of Nodoka. Everyone notices that she's not as subtle as before. The real Nodoka showed up and hit Haruna with a book right before "Ero Nodoka" could also be created. Shame, really.]
 * Even later, she creates a duplicate of Negi for the purpose of "practice pactio kisses". Naturally, the results are disastrous. Poor Akira...
 * When a vengeful spirit doll in Ranma 1/2 trades places with Akane's soul in her body (leaving Akane herself trapped as a doll) she attempts increasingly seductive approaches to get Ranma to lower his guard... so she can kill him. All but one have unforseen interruptions.

Comic Books

 * Inverted by Peter Milligan's Shade the Changing Man, who possessed the body of a serial killer who murdered his girlfriend's parents and ex-boyfriend. Unfortunately the body still possesses the killer's mind, which is able to possess the body right back. Kathy isn't clued by Shade's performance in bed because they hadn't had sex before, but when they do, she remarks on how different and 'gentle' it is. She says, unconvincingly, "But this can still be good..."
 * When Starfire was temporarily replaced by a shapeshifter from an alternate universe (or so she was at the time), the shapeshifter was much more sexually aggressive than her boyfriend Nightwing was used to. Nightwing didn't suspect anything, mostly because they'd been having problems and he was just happy that the problems seemed to have evaporated.
 * Since Empowered started wearing her suit, she climaxes "at the drop of a hat". Not That There's Anything Wrong with That.

Film -- Animated

 * Shrek: Although it never reaches a point of being sexual, Princess Fiona's reaction to Prince Charming, while he is pretending to be the yet-to-be-seen now human appearing Shrek, is constantly being off-put by his overly romantic behavior, knowing it is out of Shrek's usual character.

Film -- Live Action
"Alex Rogan: Maggie! You're never going to believe this! Maggie: (angrily slaps him) I told you, Alex! Me and my, how did you put it, "strange sexual urges" aren't talking to you anymore!"
 * The Last Starfighter. While Alex Rogan was away from Earth, he was replaced by an android Beta Unit without the knowledge of his girlfriend Maggie. When he returns and meets her:


 * Dana when she gets possessed by Zuul in Ghostbusters. "I want you inside me."
 * In Face Off, cop Sean Archer and crook Castor Troy exchange faces and lives; they immediately go about resolving each-others' family problems in their own ways. Castor, however, uses the situation to his sexual advantage. Sean's wife, Eve, thinks his romantic behavior is odd, but doesn't see through it until the switch is actually revealed.
 * When a man in the movie Slither is posessed, one of the first things he does is have sex with his wife. The wife found it a little strange he was acting so horny, but just the same liked the sex, saying something to the effect "He was like a child, exploring my body for the first time."
 * Gor from The Brain From Planet Arous is a Sense Freak. One of the senses he wants to exploit is... you guessed it. His host being engaged is to him a bonus, since he can also tap into the man's feelings for his fiance. This being the fifties, she's put off by her fiance's sudden forcefulness.
 * In the film version of X Men, Mystique disguises herself as Jean Gray to seduce Wolverine. In addition to the changed behavior, Wolverine immediately spots the ruse because of his Super Senses.
 * In the movie Dave, the President's wife figures out that her husband was replaced with a double because he looked at her legs.

Literature

 * In Parke Godwin's Hugo-nominated story The Fire When It Comes (title story of the collection of the same name), the existence of the ghost is revealed when she takes over the body of the (sexually repressed) wife during sex.
 * Subverted in Robert Sheckley's Immortality, Inc. The protagonist is in the body of another man (quite legally—long story) and is afraid that lingering traces of that man's personality taking over his own. When he meets ex-girlfriend of that man, they have sex, and the protagonist is disturbed by his unusually rough behaviour. Subvertion comes then
 * In Jewish folklore, one story tells of the demon king, Asmodeus impersonating King Solomon. While in this guise, he propositions all of Solomon's many wives (and in some tellings succeeds in sleeping with all of them). The first clue to him not being the real Solomon, is that he approaches them when they were menstruating, a major taboo. His impersonation is revealed when he tries to seduce "his" mother, Bathsheba.
 * In Journey to the West the king of Wuji was killed and replaced by a shapeshifting demon. It was a perfect imitation, except he never slept with any of his wives or concubines, which eventually contributed to his being found out.
 * In the second book of the Codex Alera, Max has temporarily replaced the incapacitated First Lord and ends up seducing the First Lady. He doesn't get caught, though Sextus does have words with Max about that when he recovers.

Live-Action TV

 * In My Own Worst Enemy, sex machine Edward makes love with alter ego Henry's wife. He's much better in bed than Henry is, leading to an affectionate Angie in the morning.
 * When Lana in Smallville becomes a vampire, this happens.
 * This happens every other episode on Smallville.
 * Bizarro acts thing way when he steals Clark's identity. Clark is pissed when he comes back and finds that no one, except Chloe, noticed anything suspicious.
 * This example is rather odd, because Clark hadn't been able to sleep with her because of his superpowers. One would think Lana would have noticed this, unless she just didn't care...
 * When Faith ends up in Buffy's body in Buffy the Vampire Slayer, she sexually taunts Spike and then aggressively seduces Riley.
 * And then has the trope turn around and backfire on her when he tries to actually make love with her instead of just getting freaky, the intimacy of the act is too much for her to handle and she flips out.
 * The Tenth Kingdom. Virginia is so mesmerized by the addictive invisibility shoes that she almost seems willing to seduce Wolf to get them.
 * In the Stargate SG-1 episode "The Broca Divide", O'Neill realises that Carter has been infected when she tries to seduce him jumps him in the locker room.
 * In the Stargate Atlantis episode "Identity", Dr. Keller gets inadvertently body-swapped with an evil Adventurer Archaeologist. The latter in the former's body (as a visual cue, the scenes have the latter's actress in, although it's clearly implied that the former is the one everyone is seeing) boards a jumper and proposes clearly to Rodney that they go to the mainland for al fresco sex, which isn't Jennifer at all. Although she does imply it at the end of the episode.
 * It's probably more of the real Keller being much less forward than the impostor was. She heavily implied that she wanted to go out with Rodney, just not that suddenly, or obviously.
 * In Alias, it's implied a woman realises her boyfriend is actually someone else who has been genetically altered to look like him as he acts differently in bed.
 * In the X-Files episode "Dreamland", Mulder gets body-swapped with an Area 51 agent. One of the behavior changes that convinces Scully that something is seriously wrong is that "Mulder" starts coming on to her. She exploits this to force him to cooperate in putting things right.
 * Same thing happens in "Small Potatoes," where the shapeshifting suspect traps Mulder in a basement and takes his form. In this case, though, while Scully suspects something's up, she goes along with the seduction until Mulder shows up and faces down the shapeshifter.
 * Scully's shocked reaction on seeing the shapeshifter change back into himself would indicate otherwise. Plus Mulder is somewhat miffed that the shapeshifter got further with Scully than he ever did (at that point in the series).
 * In Farscape episode 'John Quixote' there was a sweeter version where John realised that what was happening wasn't real when the fake Aeryn told him they should leave together and abandon everyone else. "Listen - John - I came back for you. Nothing else matters. I just - want to be with you." "Something's wrong."
 * On Heroes, Matt Parkman's body is taken over by Sylar, who promptly uses it to seduce/rape Matt's wife. She loves it (or Sylar used Matt's powers to force her to). An earlier, non-sexual example is when Sylar uses his shapeshifting powers to impersonate Sandra Bennett and file for divorce with Noah.
 * Parkman himself used his mind reading powers to set up the perfect date with his wife earlier in the series.
 * In the Star Trek: The Original Series episode "The Enemy Within", one of the first things Kirk's "evil" half does is try to rape Yeoman Rand.
 * In a non-supernatural/science fictional example, House has one of his clinic patients develop hypersexuality. Played realistically straight in that he warns a family member that the 80+ year old woman in question has had a sudden, unexplained change in behavior and needs to be examined. Good thing, too, because it turns out she has
 * Fringe: Alternate universe Olivia smooches Peter so he won't see the blood trickling across the floor from the murdered man in the bathroom.
 * Doctor Who: When Rose Tyler gets possessed by Lady Cassandra, she surprises the Doctor with a sudden kiss.
 * Hilariously, his first reaction isn't "something is really wrong here", but "Yep...still got it."

Video Games

 * Meryl in Metal Gear Solid.
 * Subverted in Eat Lead: The Return of Matt Hazard, Matt Hazard, a sentient, videogame character, is helped in his fight for survival by mousy female programmer named QA, whose ingame avatar is colored blue. She berates Matt for both his old one-liners and for hitting on her. Later, QA reappears colored red, and comparatively sassy. After a few mess-ups, blue QA finally reappears, and red QA tells Matt he has to figure which one is real. Matt, being experienced with several game genres and is, by consequence, incredbly genre-savvy, makes a comment to red QA to the effect "Oh come on. I knew you were the evil twin a long time ago."

Web Comics

 * In It's Walky!, Joyce's "evil" clone is easily identified by the fact that she merrily seduces whichever parts of the male cast is available (the resident Jerkass, as it turns out). Joyce's response... ain't pretty. Or surviveable.
 * The usual variant is inverted in the BL comic 'Paradox,' when Tommy is possessed by a soul-eating demon. Dake realises something is off because Tommy's reaction to an offer of sex is 'maybe later,' after he'd been trying to get into Dake's pants for the last couple of chapters.

Western Animation

 * In the infamous Justice League episode "The Great Brain Robbery", a notable scene occurs where Tala seduces Lex Luthor, not knowing that he's recently switched bodies with The Flash. After the situation is reversed, Tala is actually disappointed.
 * Well, they do say he's the fastest man alive. Though he was in Lex's body, and if she'd used magic, it's possible she wasted a spell.
 * Her exact words were that she wasn't used to him being so "gentle", "caring" or "enthusiastic."
 * When Homer Simpson was kidnapped and replaced by a lookalike with an obvious German accent and no understanding of Homer's personality in extended Parody of The Prisoner, it's squickily implied that his offer of "efficient German sex" is what keeps Marge from realizing anything is amiss for several months.
 * Spoofed in the parody of The Island of Dr. Moreau from Treehouse of Horror XIV: Marge gets turned into a literal catwoman by Dr. Hibbert and gets intimate with Homer. The next morning he finally notices her change when she jumps out the window to chase a bird, remarking "I had no idea! ...Though I suspected something during the sex."
 * In The Boondocks, in the episode "Stinkmeaner Strikes Back," Tom Dubois is this way with his wife Sarah, after being possessed by Stinkmeaner.