Three tendencies in role-playing styles. Any given player will exhibit each of these tendencies to some extent, although some may be so strongly oriented towards one style that the others get obscured.
Driver. Drivers are very goal-oriented; they like to figure out what they should be doing next. This might mean processing clues, strategising, intuiting out what the GM wants the group to do, or setting up a wiki to compile the information gathered over the course of a campaign. GMs love them for this, because it helps them immeasurably with the cat-herding task of keeping the action moving; other players may find them a bit difficult, because they glom up all the important plot elements. Drivers tend to have trouble with the actual playing of roles; they approach situations as they, out-of-game, would approach them. A strong Driver tends to become de facto party leader, regardless of what their character sheet says. They’re not necessarily munchkins, but when they are they’re very effective munchkins, avoiding direct confrontation with the GM but somehow getting their way. Solo adventures really bring out Driver tendencies.
Aide. Enjoy, and are good at, helping other characters shine. Since this is a character-actor kind of role, they tend to stay in character rather better, but sometimes have trouble with the limelight. Obviously, other players like them, and they tend to make excellent GMs. I’m not entirely sure how GMs feel about them, honestly; the tendency doesn’t produce very much motive force on its own, but it makes the process more fun. Aide-style play can be the result of underconfidence; but it’s also, I think, a hallmark of very mature players. A lot of the MUD regular players are in the latter camp, and it’s an honour to play with them, it really is.
Narcissist. Narcissists have figured out that the point of role-playing is Cool Moments, and they want to be the star of those Cool Moments. While Aides and Drivers could have anything on their character sheet, narcissists gravitate towards physical-action types – and usually similar characters over and over again, because they have a paradigmatic Awesome Character in their heads and don’t want to stray too far from that. Narcissists can be a real pain in the arse for a GM, because if the focus moves off them for too long they’ll derail the plot to get the spotlight back; they really don’t like to put in legwork to get the plot moving when it lags. But their glory-hunting can be highly valuable in the right situation: if the PCs are crouched behind cover exchanging round after round of inconclusive gunfire, the narcissist is going to be the one to strap on a jetpack and a belt of grenades and do something memorable. They make fairly bad GMs, over-eager to tell their own story rather than help players shape it.
Most players start out as narcissists, in large part because most players start in their teens. And most players retain a greater or lesser chunk of narcissism, which can be useful if properly managed; a great deal of game design goes into cultivating the inner narcissist for the greater good. (Primetime Adventures is, I think, a very interesting way of doing this, because it basically gives every player one session per campaign where they’re given free license to indulge their narcissism, and the other players have to help.)