I just saw a two-person play where one character did the majority of the talking and the other seemed to exist only to give the other character someone to talk to. I've seen this happen in books and TV as well, and sometimes a normally strong character will take this role temporarily. Usually the conversation plays out something like this:
A: < Talks for 3 minutes about ducks >
B: Oh, interesting, so you're saying that ducks can fly and swim?
A: Exactly! < Talks for 3 more minutes >
B: So what about geese? I've heard about them too.
A: Oh, those are similar, but there are some differences. < Talks about geese >
And so on and so forth. Character B never really contributes anything meaningful to the conversation and seems to only be there so it's not a 90-minute monologue. I would certainly view this as Bad Writing, and something that would be more appropriate in an children's educational video.
Is there a term or trope for this type of character or scene?