How to improv a fake identity on the spot?

-Say you meet someone you're never gonna see again and just want to screw around and pretend your someone else like add an accent, walk differently, think differently, and talk differently than normal. Or you just want to have some laughs with your friends by acting like a sterotype. Or last but not least, you did something that you regret, people come and see whats up, you need to talk your way out of the situation. How can you improv these things or identities quickly and on the spot?Be someone you've always wanted someone perfect. or test people talk with a lisp or have a scratchy voice ad see if people judge you