As others have also noted, for me it depends on the game/genre we're playing in.
There are games where I get very much the mode of "method-acting" roleplay where I get fully subsumed by the character and the situation, including attitude, mannerisms, and speech. (Even going so far to inadvertently ask the GM a question in the same way my character would talk...)
For other games, especially those with rules and modes of play that emphasize them, I'm more separated, letting the game's impetus (and sometimes suggestions from other players) guide things. It always remains true to the fiction and to the character*, but it's less extemporaneous and narrowly focused to the character's immediate perceptions and allows bringing in grander arcs and considerations.
I don't think I prefer one more than the other, they're both awesome in their own right and create excellent fun and excitement and cool/satisfying stories**. I love the variety in how we get to play the games and the variety in how we get to play the roles.
* (it's a huge peeve of mine how many modern blockbusters have characters wildly vacillate from scene to scene, completely subservient to the moment at hand that the director thinks will be 'cool' but never to the grander narrative as a whole nor the characters as real sapient entities.)
** (I now feel compelled whenever I use story on this site to point out this does not mean that the thing is pre-written and forced upon the players/the GM/anyone at all, nor is it purely dissasociated meta-whatever. This is the story that is emerging through play that the group can recount later, irrespective of the method of play.)