Makes Perfect sense! I do the EXACT same thing!!!
Though I think I find myself doing it mostly with people I meet here, or talk to only via chat or messaging usually... not those I meet or see face-to-face...
It seems most of those things I would be too afraid or anxious to speak out or be, irl...

I act like someone with certain traits I want to have for the situation, and be; but cannot even imagine it coming from the real me at the same time... So I in some way, start conversing as if it's coming from a different character...
Seems I see myself much lesser than I am, a part of putting myself down before others do it, easier to handle...

Fear of rejection in me is great, but fear of rejection of what i expect and hope of myself to be so as to not be rejected, is greater...
So I feel, maybe this behaviour is kinda like testing, whether the person I feel I want to be (sometimes just for that moment), is in fact the right person to want to be or not?
So if I make it feel foreign to myself, then even if that trait or person does turn out to be unacceptable or something, or I end up failing myself or the opposite person, the blow feels lesser than it would otherwise? idk...

My apologies if I've been too repetitive, or if the post is too long, not very good at concise writing.
Can't help you much, trying to understand it myself... But just wanted you to know you are not alone in this.