You accidentally run across someone you admire. Could be a singer, actor/actress, politician... You would...
Depends - did I run across them with my car?
I'd say hi.
I don't care who it is... If they're open to talk - I'll talk! :D
Other not sure
I met Eartha Kitt and all I could stammer was "Uh, I think you're neat" and I freaking idolized her.
I think I could act normally but cant guarantee it.
It'd depend on who. But most likely I'd start chatting.
Imagine Obama. Could you just casually chat?