Zinc I can speak in a smaller or larger group too, or be on stage or such. It depends on the people and what you have to say, somewhat. At least for me. (I can have anxiety before also, sometimes, but it may be nonexistant while actually doing it or later.)
I think the difference is whether you are expected to say something and the environment is structured, or not. Like what Helman said.
And whether it's actually interesting, or not. I can get bored to hell with some 'bar conversations'... (My gorgeous cousin had one guy guessing her telephone number for ten minutes?!!)
There's at least one other guy here who is okay with being on stage and shy in RL.. I think many actors or musicians can be shy in RL too.. Partly that's what maybe draws us to acting and music.. To 'get out of the shell'...
I was more comfortable at parties after I read some stuff about how to act in such situations and had some 'scripts' or ideas to try.. I later realized I have nonverbal learning problems, so reading about stuff is EXTREMELY helpful to me!! I still don't go to bars much though, lol. It just feels like a waste of time, somewhat.. There are people who just never go, or rarely.. And they can be cool and gorgeous and have 'a life'... It was a big eye-opener for me when a girl just said, 'I don't go to parties much'.. She had a boyfriend and everything... (Maybe she didn't go because of that too, hmm? Some bfs can be a bit possessive, or afraid girls would meet other people then..)
Sorry, I'm rambling a bit. Thing is, as people with sp, we might think we 'have' to go to parties/bars/clubs and enjoy it there, there is no written rule though.. (And some of those 'fabulous partygoers' may get troubles with alcoholism or drugs or depression or even commit suicide later on. I've seen it happen.)
You can just go and expect to 'not have fun', and to just 'suffer through it, get yourself out there', not expecting anything or any 'fun' - and then maybe you just will!!