Summary
Have one player step out with a physicality and short statement with a nice voice choice. From there, have the other players say things that are now also true about this character, have that character simply agree or justify and let it slowly shape them into a more fulsome character.
Example
I might step out with a classic ‘nerd’ stance and voice and say something about Digimon being superior to Pokemon. Because we’re all so different and can offer a lot of subtext in our performance, maybe there’s more there… someone could say ‘you hate bananas because they’re either too ripe or not ripe enough.’ I would agree, and now maybe we can build on this character who is very particular or needs things to go a certain way.