Role-playing games are social games. It is by talking to each other—sharing our imagined content—that we establish the environment of the game.
Anything that is not shared, obviously, does not become part of the collective experience of the game. That doesn't mean that things you keep to yourself aren't important, or that they don't contribute to how you portray or even feel about your character. But unless you share what your character is feeling or thinking, others in the group can't pick up on it and act upon it. Nor are they going to be able to fully appreciate your character's actions taken as a result of those feelings or thoughts.
All that is a roundabout way of getting to what I consider to be the most essential, foundational skill in role-playing: listening. And yes, you're right. That's not such a deep observation. But sometimes it's helpful to review the fundamentals. In fact, I'm bringing this subject up because I needed the review.
What gets your fellow players jazzed about their characters? What sorts of conflict or bits of setting or color get them excited? What sorts of situations get the GM to turn it up to 11? Also, what sorts of things make your fellow players tune out? Being aware of these things is the first step toward helping your group achieve amazing play consistently.
Whether you're a GM or one of the other players, do your best to focus on the other people at the table and what gets them excited. If you can keep a running tally in your head, great. If not, make a note of it. I find that I need to make notes if there are more than four players other than myself.
Use those notes as you play. When it's your turn to do something, try to ensure that your actions activate the interests of at least one other person in your notes (or your running mental tally). And, of course, do your best to avoid actions that will cause others to tune out.
Simple, right? If you've got any tips or tricks to aid this process, please share!