If someone in a group says nearly nothing all the time, let others discuss things, just waiting for the perfekt moment to throw in a gag, you can call this behavior humor snipering.
Most of the time followed by a surpised reaction or laughter (not slaughter, that's the difference to real snipering).
Humor snipering is regarded as a high art by some people.
Also known as Gag snipering
A long discussion about a problematic family structure is going on, about what the problems in the family are and why there are so many suicides.
Person 1: As you can see, the communication has never been very good in this family. Suicide is no communication.
Person 2 (nothing said so far): Yes it is, it's non-verbal communication!
(Followed by a lot of breath-ins and gasps, laughter too)
What Person 2 did was "humor snipering"