Screw suspension of disbelief. You're really into theater now, you want to figure out all about how acting works, and so you want to learn out how to do all sorts of characters to develop a good range, no matter if you're actually good for casting into one or not. So now you're trying to figure how to do the confident self-affirmed character, starting from the nuts and bolts. Figure out how they use their voice, how they move themselves, what body language and stereotypical interactions they use in various stock situations and so on. You're not being in a social situation yourself here, you're figuring out the mechanics for making a theater scene come together, with yourself as one part of it.

Also maybe look into some actual books on improv theater that have been recommended here occasionally?