Finish Dialogue in Unison

One character starts saying something, and while completing the sentence, another character (or several) join in.

"Alice: I just know this is going to be-- Alice, Bob, Carol, Steve, and Fitzwilliam: The Best Trope Ever."

May be done sarcastically (the first character says X so often that everyone can tell what's coming), or as a sign of group solidarity (especially common with the Catch Phrase). This also often happens with Eureka Moments that involve two or more characters of similarly high intellectual level. It's played for comedy if the characters aren't really that smart, or they're coming to same ridiculously outlandish Insane Troll Logic conclusion by complete coincidence.

Gags aplenty exist for this one—the people say different things, or there's an Expospeak Gag or something. Or one of the characters yells "Jinx!" There's also usually also a coda, because you can't let something like that go.

Closely related to Finishing Each Other's Sentences. Could be related to Are You Pondering What I'm Pondering?. See also Speak in Unison, for when characters do more than just finish the thought at the same time.

Film--Animation
"Flint: Sentient food? That's impossible! Sam: Unless its molecular structure's mutated into super food! Sam and Flint: That's been genetically engineered to protect the FLDSMDFR!"
 * Used with Sam and Flint a couple of times in Cloudy with a Chance of Meatballs, first near the beginning when they're deciding what to "make" for breakfast the next day, and then near the end:

"Sultan: The law says you (Jasmine joins in) must be married to a prince (Jasmine trails off) by your next birthday."
 * Aladdin has a variation, where Jasmine trails off near the end:

Film--Live Action
"Madison Lee: I never wore Kevlar till I took three in the chest... Natalie, Dylan, and Alex: On the De Soto case - we know!"
 * In the David Tennant film of Hamlet, Polonius, Laertes, and Ophelia all recite the end of Polonius's "never a borrower nor a lender be" speech in unison. It's very clearly the sarcastic version.
 * In the movie Chitty Chitty Bang Bang, the avid traveller, Grandpa Potts, recounts one of his stories from India, saying "I got up this morning and I shot an elephant in my pajamas. How he..." and his son Caractacus, with his two children, Jeremy and Jemima, finish the joke for him: "...ever got in my pajamas I shall never know." Grandpa nonchalantly replies, "You've heard it before." And everyone laughs.
 * In the movie Baby, the couple that comes across Baby the dinosaur eventually learn to repeat their Jamaican pilot's oft-repeated Catch Phrase "No problem!!"
 * "Orange Mocha Frappucino!!"
 * Charlie's Angels: Full Throttle:

"Billy and other inn patrons: Now isn't that a story worth the hearin'? Muppet Pig: I' was the first dozen times we heard it."
 * Duncan and Jason throughout Mystery Team. Charlie tries to do this as well, but fails with one exception: "Chocolate Milk."
 * Muppet Treasure Island, right after Billy Bones finishes telling the story of Captain Flint's treasure...

Live Action Television
"Kids: What (or who) do you think's in the burgers?
 * In one Scrubs teaser, the entire cafeteria finishes Elliot's old jokes for her, which causes her to shriek "STOP FINISHING MY AWESOME JOKES!!!" (And Zach Braff to react to the unexpected shriek with an "Oh my God!")
 * Done Once an Episode on You Can't Do That on Television, when they realize the current skit is Just the Introduction to The Opposites.

Barth: Duuuuuh... I heard that!"

"Hawkeye: He whistles louder-- All: When he's bluffing!"
 * Played for laughs on Thirty Rock. Jenna and her boyfriend lampshade/outright parody this. The boyfriend suggests they announce their ideas for Halloween costumes at the exact same moment, and just "see what happens." Cue them both rattling off a bizarre idea for matching costumes (Natalie Portman and Pittsburgh Steeler—two 'black swans!') using the exact same wording.
 * Mash combined this with a Eureka Moment when Hawkeye realized what Winchester's poker tell was:


 * Noah's Arc: Happens numerous times to the point of being a Running Gag, three times in the first episode alone. Typically one of the four main cast starts the sentence and the other three finish it together.
 * A Saturday Night Live sketch from the 80s involves a TV interview with two members of the American Communist party, who happen to have been friends since childhood. Every time the two of them say the same thing in unison, they immediately shout, "Jinx, buy me a Coke!" and then launch into an elaborate playground song-and-dance routine.

Theatre

 * The second time in Of Thee I Sing Diana Devereaux uses her Catch Phrase "I'm just going to throw my arms right around your neck," all the other girls chime in with her.

Video Games

 * In World of Warcraft, before the final battle with Cho'gall starts, his two heads get into an argument over the crazy one interrupting the sane one ("I am trying to speak here!"). However, by the end of the speech, they are talking in unison again, until they deliver the very last line together: "The end has come!"

Western Animation
"Supergirl: It's a shame our Justice League comlinks don't work here-- we could track Green Lantern by his transponder. Come to think, we still could if we had a-- Supergirl and Brainiac 5: Quantum tunneling RF transponder! Brainiac 5: Hold that thought."
 * Used a few times in Justice League Unlimited
 * In the episode "Far From Home," this is used to show how well Brainiac 5 and Supergirl get along. (They're trying to find their teammates by using Supergirl's earpiece.)

"Shining Knight: I'd slay the ogre Blunderbore all over again rather than put myself on display in this manner, even though that ogre turned out to be-- Vigilante, Stargirl, Green Arrow, S.T.R.I.P.E.: Morgan Le Fay. Green Arrow: We've all heard it. Shining Knight: (miffed) 'Tis a good story."
 * "Patriot Act"

"Chris: Oooh ...oooh! What if they just bought a new wood stove and Red Hynie Monkey says, "Boy, it's so hot in here because we just bought that new wood stove... Both: and we're sweating, and our hands are all slippery and that's why we can't get the card house to stay up!" Peter: Thank you, Chris. We have lift off!"
 * Strange Minds Think Alike on Family Guy when Peter and Chris think of ideas for "Handiquacks"

"Twilight Sparkle: ...No!... Rarity: ...I've waited all my life... Fluttershy:...for this moment... Pinkie Pie:...and I'm not going to... Applejack:...let it slip by!... Rainbow Dash:...If it's the last thing I do... Twilight Sparkle: ...I'm gonna make this... All:...the best night ever!"
 * An episode of Clone High had Abe and Gandhi repeatedly do this when designing a mechanized spoon, fork, and knife combination. Then it gets subverted on the last shot when Abe and Gandhi don't say the line in unison, and Abe angrily tells Gandhi something along the lines of "c'mon, we rehearsed this!"
 * Kim Possible: Kim and Ron would frequently find themselves saying the same thing, usually followed by Kim saying, "You owe me a soda." At the end of the fanfic Reunion, Kim and Ron, on their honeymoon, say "I love you" in unison, after which Kim tells Ron, "You owe me a baby."
 * My Little Pony Friendship Is Magic episode "The Best Night Ever":

"Choo Choo: He was in front of a display with a big sign that said "Watch This Space." T.C.: So? Choo Choo: So, Benny was just standing there... Choo Choo and T.C.: Watching the space!"
 * The Top Cat episode "Rafeefleas" has Choo Choo explaining to T.C. what Benny the Ball was doing at the museum: