An interlocutor is someone who participates in a discussion or conversation, sometimes as a go-between. If you and your friend are in an argument and aren't speaking, a third friend could act as an interlocutor, delivering messages back and forth.
Interlocutor traces back to the Latin word interloqui, meaning “to speak between.” We use interlocutor either for "a conversation partner," or for "a third party in a conversation," like a middleman who speaks on behalf of an organization. It's also an old term for the guy in a minstrel show who questions all the other performers as a way of moving the act forward. "What's that you say? A funky dance? Go on and show us then."