Walk In, Chime In

Alice and Bob (and any number of others) are having a conversation. Suddenly, Chuck walks in and responds directly to something someone has said, even though he wasn't in the room to hear it.

This trope only applies if the following conditions are met:

Chuck was not eavesdropping and/or had no way of overhearing the conversation prior to stepping in (this also covers such things as: conversations in an open area (unless Alice and Bob are genuinely alone); conversations broadcast on a loudspeaker or similar system; or Chuck being told of the conversation after the fact and resuming it much later);
Chuck is clearly responding directly to what someone has said, for instance answering a question; it's not just a coincidental dialogue.

This seems to be an inverted application of the Rule of Perception: because the audience has heard what Alice and Bob were saying, so too did Chuck. The point is that Chuck had no way of hearing the conversation, yet he enters the room and joins the conversation like a LEGO brick.

Compare Speak of the Devil. See also Realistic Diction Is Unrealistic.

Examples of Walk In, Chime In include:

Anime and Manga

In Mahou Sensei Negima, Ayaka does it over the phone. No one supports a relationship with Negi, unless it's her relationship with Negi.
- Then she has an active conversation with the mostly silent Zazie, to the shock of others. Again over the phone.
In the last episode of Magical Girl Lyrical Nanoha A's, when Fate protests that sealing the Book of Darkness will also seal Signum and the others, Signum arrives with Shamal and Zafira, and says that they will be able to live on.

Comic Books

There was a particularly egregious example on Teen Titans where Dr. Light is in the middle of a vast, empty snowfield talking to himself — the Titans drive up in the T-ship from miles and miles away, jump out, and Robin responds to what he was saying.

Dr. Light: And out here in the Arctic circle, no one can stop me!
Titans zoom up
Robin: Wanna bet?

Film

In Spaceballs, during the wedding at the end: When asked if she takes Prince Valium to be her husband, Princess Vespa hesitates for a couple of seconds, giving Lone Star plenty of time to enter and answer the question for her.
Lampshaded in Muppet Treasure Island: "How does she do that?"
In The American President, President Shepherd is in the Oval Office talking to his chief of staff about going over to his girlfriend Sidney's house and groveling until she takes him back (she'd walked out on him after he made a deal to kill a bill Sidney worked hard to get the votes to pass, in order to get the votes for another bill to pass). Suddenly, she opens the door and walks in, telling him to "stay off Dupont Circle when you do; the traffic is murder at this time of day". She couldn't have heard him, because the Oval Office is soundproof.
In Kiss Kiss Bang Bang, this is one of the many things that make Gay Perry awesome.

Harry: What is it out here with these women?
Harmony: Oh please, Harry, they're no different from anywhere else.
Harry: Yes, they are. These are damaged goods, every one of them, from way back. I'm telling you, you take a guy who sleeps with 100 women a year, go into his childhood - dollars to doughnuts, it's relatively unspectacular. Now, you take one of these... gals, who sleeps with 100 guys a year, and I bet you if you look in their childhood, there's something rotten in Denver.
Harmony: Denmark.
Harry: That too! But it's abandonment, it's abuse, it's, "My uncle put his ping-ping in my papa!"... and then they all come out here! I mean, it's literally like someone took America by the East Coast and shook it, and all the normal girls managed to hang on.
Harmony: OK, everyone who hates Harry raise your hand!
[All the women in the club raise their hands]
Perry [entering]: See that? Obedient little bitches too.

In the film of Scott Pilgrim, Scott's sister breaks into his conversation with a practically-passed-out Wallace Wells by cellphone to call him out on double-dating. When asked how she knew, she tells him Wallace told her; pan down to Wallace, who appears to have completely passed out but has still somehow texted the information to her.

Live-Action TV

In one episode of Night Court, the group is running interference for Harry with a pushy bureaucrat, trying to cover Harry's absence (preparing a suicidal stunt); when at last the man demands to know where Harry is, the gang hems and haws a bit. And then Harry steps into the office and says "He's right here!"
Monty Python's Flying Circus. What, you want more detail? Geez, I wasn't expecting a Spanish Inquisition!

DRAMATIC MUSICAL STING
No-one expects the Spanish Inquisition!

Maynard G. Krebbs did this all the time on The Many Loves of Dobie Gillis. Someone (usually Dobie's father) would be talking about lazy, good-for-nothing teenagers, and Maynard would walk in and say "You rang?"
Gibbs does this on NCIS all the time. Tony attempted to copy him once by eavesdropping, thinking this was how Gibbs does it (it isn't, as Gibbs then goes on to demonstrate). You could probably rename this trope "Gibbsing". Somehow he's able to walk into a room with exact knowledge of the preceding five minutes of dialogue. At least once. Every. Episode.
In the first season finally of Heroes Claire doesn't want to shoot Peter, saying that there must be another way. Peter says there isn't one. Nathan then comes flying in from a distance, and says to Claire "Yes there is".
On Friends, Monica and Chandler are debating about whether it's OK to harmlessly flirt with others while in a relationship. Chandler argues that it's OK for him but not Monica, saying "There's a difference, you are way hotter than I am," and Joey, passing by, pipes in with "True story!"
On Will and Grace, at Karen's wedding, Jack offers Rosario an hors d'oeuvre by saying "Shrimp?" The diminutive Beverly Leslie, passing by, retorts "Queer!"
Angel does this as a Running Gag—plausible due to his vampire hearing.

Gunn: "He's Angel. He does that."

Western Animation

A variation on Extreme Ghostbusters. During an episode where Eduardo is trying to bend the rules on a project, Kylie and Roland are arguing with him, with Garrett doing the chime-in:

Eduardo: ...cut a few corners...
Kylie: Yeah, and lie,
Roland: Cheat,
Garrett: Steal. What are we talking about?

Batman does this all the time in animated series — a villain will be monologuing, then Bats will crash in through the skylights and deliver a One-Liner in response. Except that for this one, we can assume he was eavesdropping.
The Avengers: Earth's Mightiest Heroes, in Gamma World, pt 2, the Leader asks Thor who will stop him.

The Hulk crashes through the ceiling. "I will.