Begin at a decision, not at the world's beginning
Ask what your character needs from this encounter. Then supply a setting and a role that make that need understandable. A clear reason to act gives the other participant something to work with before you have written pages of background.
Try a starter prompt such as: 'You run a small radio station, and I am the producer. Tonight's guest has cancelled ten minutes before broadcast. Stay in character, ask me one question at a time, and let me choose my own actions.' Change the roles and immediate problem to fit the character you selected.
Explain how you want the exchange to read
State boundaries and preferences as concrete requests. You can ask for less narration, a more gradual pace or shorter spoken lines. When a reply is not what you wanted, describe the alternative so the character has a direction to follow.
Make it clear which words are dialogue and which describe an action. Keep your point of view steady enough that a reader can identify who is doing what. Moving between first and third person without explanation can make the participants' roles harder to follow.
Separate lasting background from the latest development
Use a Lorebook for established world knowledge rather than putting every new event into the character's permanent personality. When a scene moves, briefly restate the time, location and unfinished objective. That recap can reconnect the next exchange to what is happening now.
A Rulebook is useful when you want to track consequences or changing story state. It is optional for an ordinary conversation. Add tracking because a particular story benefits from it, not because every roleplay needs a set of meters.
Questions
Can I play a fictional relationship with an AI boyfriend or girlfriend?
Yes. Establish the relationship with an adult fictional character you choose or create. It remains a fictional exchange, not a real person's identity, permission or commitment to you.
Does the opening greeting determine the whole plot?
No. You can introduce decisions and events after the opening situation. A well-defined character gives those changes a consistent point of reference without fixing every outcome.