interlocutor
formal A person who takes part in a dialogue or conversation.
(from here)
If Lexico says some word is "formal", then it means it's formal AF (it flags them as such much rarer compared to, say, the Longman dictionary). I don't want to pose as some aristocrat without a reason. How can I express the same idea using everyday language? Don't suggest 'speaker', it doesn't fit (it's only applied to spoken speech, as I understand it, I need a broader term).