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eroock
Since: Sep, 2012

I've seen this more in Law and Order:SVU but I'm sure that must happen in other shows.
Two or more characters are chatting about some case or problem. At a given point, one of those character says something crucial and then a third (or fourth or fifth) character that wasn't there, enters the scene with a very useful/timely/brightful/witty comment.
For example:
Olivia: I'm gonna stay to drive her to Rikers in the morning. Elliot: Look, she made her own choices. Olivia: Doesn't mean that I have to feel good because she's going to jail for murder.
Suddenly we hear the voice-off from Casey Novak:
Casey: She's not!
They turn to look at Casey, coming from the shadows with a tape in his hand.
Casey: Her lawyer sent this to me.
The characters doing this always came from outside of the scene, looking as if they managed to hear the last bit of dialogue and they are able to intervene with surprising news.
Does this trope exists? Is it a broader trope or is only a SVU writers trite resource?