I love it when people can go from zero to 100% acting drama for a bit/improv/etc. My favourite example, of course, is pretty much anything Brennan Lee Mulligan says or does.
Like, some default-char-gen-lookin' guy, someone you'd expect to talk about fucking stocks or something just on sight alone, slings a flawless 'distraught, soulwrenching panic about an invading army of pickles'.
That perfect combination of honed, realistic seriousness combined with the absolute silliness of the situation, and they go they whole hog into becoming the character for like, 5 seconds, and then they're out again.
LOVE that shit.