Hi there. It’s always safe to just use the gender-neutral “they”. Also can use nonbinary person, as that is someone that doesn’t identify with sex that was assigned to them at birth. Hope I can help.

If you are speaking of NPC’s (non-playable character) then they are usually recognized by their title. “Inn keeper” “Trainer” “Neighbor” “Angry child” etc. You could use “they” and “you” as pronouns. “How are you?” “Are you okay?” “What are they doing?” “They are welcome to join us.”

Yes I agree, "interlocutor" is very formal and not something you would want to use very often. It's interesting, I notice this word being used by Russian speakers. It must be the exact translation of a common word in Russian. A one on one conversation is a "dialogue", so you can research "how to write dialogue" online and you may find some useful websites. It's difficult to write dialogue if you want to avoid gender pronouns. Amateurs like myself often use "he said" and "she said" when describing a conversation, but proper writers can find ways around this problem. For example, you could refer to the occupation of the person to start the conversation and then avoid the amateurish "he said/she replied" tags at the end to make it a nice clean dialogue. The cop said I was in big trouble. "Why? I've done nothing wrong!" "You ran a red light." "It was yellow!" "The trunk of your car is open and it's stuffed full of money." "It's a rental, must be someone else's." "There are two people in your back seat with shotguns and ski masks." "Never noticed. Like I said, it's a rental."