The Man in the Mirror Talks Back

You have Mirror Monologue, where the character is talking to his reflection.

And then you have this trope, which is when the reflection is the one doing the talking (or, occasionally, moving).

The difference? The Mirror Monologue is realistic (physically, if not psychologically); nothing fantastic is happening, so we can pretend we're objective observers. Many versions of this explicitly have the mirror moving in a completely different way than the 'real world', thus bringing it over into either the fantastic, or we're explicitly seeing things through the character's eyes. Those that don't do the 'mirror isn't reflecting the real world' thing via special effects of one sort or another but still qualify are using editing for much the same effect (see, e.g., the first Spider-Man movie, or the Gollum/Smeagol dialogue from film version of The Two Towers).

Usually used, much like the Mirror Monologue, to make visual an internal dialogue, either between different aspects of a character (such as a man and his conscience) or between different personalities sharing the same body.

Somewhat of an inversion of Mirror Routine; there, it's a different person acting like a reflection; here, it's a reflection acting like a different person.

Anime and Manga

 * It happens more than once to Allelujah in Mobile Suit Gundam 00. For example, in the last episode of the first season, he has a conversation with his other personality, Hallelujah, that is reflected in his Mobile Suit's screens.
 * One episode of Betterman has this happen to the entire main cast besides Keita and Hinoki. Each reflection delivers a "The Reason You Suck" Speech, but is eventually defeated by a Shut UP, Hannibal counter-argument. For the record, these were hallucinations somehow tied to Algernon.

Comics

 * A comics gag seen in both Doonesbury and Bloom County. (B. Breathed later said he inadvertently ripped off the joke from Trudeau).
 * Some early 60s Perishers strips had a Running Gag where the neurotic Wellington would pour out his existential anguish in front of his mirror, and his reflection would make some sarcastic remark once he'd left.
 * Used in FoxTrot, of all places, when Jason complains to his reflection how unfair it is that his family want him to donate his allowance to hurricane relief rather than buying a newspaper comic collection. His reflection agrees and goes on about how unfair it is of the family to assume that the needs of those whose homes and lives have been destroyed outweigh his desire to giggle for 30 minutes (or 45 if he reads slowly). Jason's final comment is "I'm not sure I like what I'm seeing".

Film
"Ash: I'm fine... Reflection: I don't think so. We just cut up our girlfriend with a chainsaw. Does that sound...fine?"
 * Gollum talks to his reflection in water in the film of The Lord of the Rings: Return of the King, as part of the Gollum-Smeagol conflict.
 * From the first Spider-Man movie: Norman Osborn had several conversations with his alter ego in a mirror
 * Which is itself influenced by a similar scene in the 1994 Spiderman series, where the Goblin convinces Osborn to let him out in a similar fashion.
 * Happens to Ash Evil Dead 2, with the added twist that his reflection even leans out of the mirror to grasp him:


 * The Muppet Movie. The rare non-psychotic instance in which Kermit's inner self materializes to counsel himself in a true Heartwarming Moment.
 * The Muppets has Gary and Walter sing "Man or Muppet" with their respective reflections. For this song, Gary's reflection resembles a Muppet, while Walter's resembles a human.
 * In Twenty Fifth Hour Edward Norton's reflection talks back to him for the main monologue of the film (the reflection monologues, not the character).
 * Mary Poppins features this during "A Spoonful Of Sugar"; at first, the reflection is echoing her lyrics, but then the reflection starts showing off. ("Cheeky!")
 * In Zoolander, Derek Zoolander works through his confusion over failing to win the model of the year award by asking his reflection in a puddle "Who am I?" His reflection answers, "I don't know."
 * In The League of Extraordinary Gentlemen, Jekyll and Hyde berate one another by way of mirrors.
 * Rockula has recurring scenes where Ralph converses with his reflection, who moves independently of him and is implied to even have a life of his own. It's never really made clear if this is all in the character's head or if this is actually happening - He is a vampire in a film where Our Vampires Are Different.

Literature

 * In Peter David's Tiger Heart, the main character has nightly visits with The Boy by communicating with him through the mirror.
 * Happens in And Another Thing thanks to the Sirius Cybernetics Corporation giving mirrors "Genuine People Personalities".

Live Action TV

 * Saturday Night Live examples:
 * Once when Mick Jagger was the musical guest he did a sketch where he talked to his mirror self, played by Jimmy Fallon.
 * This got a Continuity Nod and a twist when Jimmy Fallon guest-hosted in 2011 and did the same basic sketch with Andy Samberg playing his mirror self.
 * Averted when Prince was musical guest during the time that Fred Armisten had as a recurring sketch "The Prince Show" where he played Prince having a talk show. They did a mirror bit but he just looked at himself. Could be a Subversion (in that everyone expected a mirror bit so they did that), or that they wrote a mirror bit and Prince nixed the idea, so they had to replace it.
 * A skit on The Muppet Show had Gonzo singing "Act Naturally" when his reflection suddenly starts singing along and then arguing with him. (You know it's unusual when even Gonzo is shocked by it.)
 * Similarly on Muppets Tonight Gonzo performed "Dancing With Myself" in a mirror room. In this case the reflections exited the mirrors.
 * In the beginning of the Dinosaurs episode titled "How to Pick Up Girls" Robbie has a borderline argument with himself in the mirror on the inside of his locker door.
 * Niki and her alter-ego, Jessica, would often interact this way on Heroes, with the dormant personality on the other side of the mirror.
 * The Twilight Zone TOS episode "Nervous Man in a Four Dollar Room". A weak, cowardly gangster's reflection tries to talk him into changing his life.
 * Hannah Montana: Miley talking to her Alter Ego.

Newspaper Comics

 * Subverted in Pogo. It looks like Porkypine's reflection is talking back to him, but it's actually a bug hiding behind his mirror playing a prank on him.

Video Games

 * In Sly Cooper 3, once enemy-now-ally Panda King has a discussion with his former self from the first game. Panda King needs Sly's help saving his daughter, but is still angry over his previous defeat. The two halves, anger and humility, reconcile when the game-3 Panda King points out that the two can work together to save Jing King, and become the father she needs. ("The yin..and the yang?")

Web Comics

 * In Eerie Cuties, there is a demon which possesses people who look at it in a mirror. It can talk as well.

Web Original

 * The second strip down, as the Comics Curmudgeon notes, is an example of either this, or Fridge Logic.

Western Animation
"The Mask: But...I didn't do it. (stares at reflection in a nearby window) Reflection: (leaning out of the glass) How do you know? Maybe you have one of those split personalities."
 * In the Classic Disney Short "Tomorrow We Diet", a fat Goofy is lectured on good nutrition by his reflection.
 * In another Disney short, "The Golden Touch", King Midas brags to his reflection that he will turn everything to gold, and the reflection applauds him. Later, as he realizes he's Blessed with Suck, he asks his refection if he's doomed to starve to death. The reflection turns into the Grim Reaper and nods yes.
 * In an episode of Fosters Home for Imaginary Friends, a feverish Bloo halucinates that his reflection is talking back to him.
 * In an episode of The Mask animated series, the Mask does this after being framed for stealing cookies from orphans and doubting his own innocence. Of course since he's a cartoonish Reality Warper:

"Felix's Reflection: Felix, ya look like a million bucks! Felix: Thanks, Felix. Wish me luck, man! (leaves) Felix's Reflection: (sighs) Poor mook don't stand a chance."
 * Pinky and The Brain: Pinky doing the Are You Pondering What I'm Pondering? thing alone, because Brain isn't there.
 * Appears in a few episodes of SpongeBob SquarePants. In "Wet Painters", SpongeBob is telling himself to be brave, but the reflection says "You're on your own" and leaves. In "Something Smells", the reflection breaks the mirror to get away from SpongeBob's bad breath, leading him to believe that he's Mirror-Cracking Ugly. Another example involves Patrick literally punching out his own reflection after it gets into a short argument with him.
 * In The Super Mario Bros Super Show episode Mario and Joliet, A Romeo and Juliet parody, King Koopa learns that Princess Toadstool has halted the feud (Which he started!) between the fighting families, and as a result, he can't sell them weapons anymore. His reflection tells him to just kidnap the Juliet and blame it on the Romeo, setting the families against each other again.
 * Happens in Jimmy Two Shoes, when Jimmy is feeling guilty about running off and not standing up for Beezy.
 * In the Musical Episode of Daria, Quinn sings in front of a mirror while trying to decide what to wear. Her reflection tells her she could pick any outfit and still look attractive.
 * My Little Pony Friendship Is Magic sees a neurotic Twilight do this in "Lesson Zero", talking back-and-forth with her reflection in a puddle about her inability to come up with a friendship report for Princess Celestia.
 * In The Twisted Tales of Felix the Cat...after Felix's reflection reaches out of the mirror to help spruce him up for a date, no less. (Not surprising, since the show runs on 100% old-school cartoon logic.)


 * The Grinch's reflection in The Grinch Grinches the Cat in the Hat persuades him to be not so very nice.