Role-playing noobie need help soon

Hello, so I found someone who is into role-playing. They roleplayed before in a previous fwb, and is now looking for a new one. It is a very specific role-play of 2 fictional characters. I never role-played in any capacity before, I don’t even dirty talk. I am as vanilla as they come. I usually try to be as accommodating as I can to someone’s kink so they have a good time, I do not have any kinks. I wanna do a good job. I will look nothing like this character and it will depend on my acting and my improv of dialogue. What can I do to prepare myself? I know these two character kind of well and I know what kind of direction they want me to act kind of. I do not think I can come up with dialogue on the fly

Edit: I mixed up fictional and nonfictional. they are both fictional characters