The rule book is there to resolve conflict, mainly in terms of combat. If you're familiar with the kid's game of cops and robbers, it's to make sure there's no arguments about "Bang! I shot you!" "No, I should you first!". The majority of mechanics are of this nature, and the rest of the book is less rules than a description of a fantasy world for players to build off of and improvise within. In general it's fairly boisterous, and the communal nature of the game means there aren't a lot of gaps. You can do your thinking during the times other players are talking about their decisions or when the monsters are acting or when the DM is explaining, so if you're playing with people who are experienced there aren't a lot of long pauses. Watching from the sidelines is pretty unexciting because most people, while they put some effort into acting, aren't that great, so if you lack the emotional connection with the characters and situations and achievements it's just not that good. Re: Shy outcasts. A lot of shy outcasts really enjoy the opportunity to act like NOT shy outcasts. DND is normally played in a safer environment where social experimentation is not just encouraged but pretty much required. Pretty much no one CHOOSES to be a shy outcast so much as they're forced to inhabit that corner of existence by everyone else. Being the center of attention of a bunch of people who you respect and who respect you is a lot more pleasant than being the center of attention of people who are primed to mock and belittle you.

My impression, from idly watching sometimes at a science fiction club, is that it's fairly boisterous, and few watch from sidelines (certainly I didn't understand what was happening, although if I had known the rules maybe I would've'd a better chance).