That seemingly quite fellow who "crouches in the brush" of a conversation, standing in the background of the crowd, waiting patiently for the perfect moment to say something hilarious.
synonym: Opportunistic Comedian
Ray: "The conversation our group was having was rather dull until Danny leaped in with a colorful plethora of 'that's what she said' jokes."
Debbie: "I know. He's usually so quiet too. I guess he's a bit of a social tiger."