I third @WTFE
I want to have a fun, entertaining environment. What that means... varies with the situation, the mood, the character, my play-group. As I play with people more I get an idea of what they enjoy, what they don't. I don't check everything with them but if something is off from our usual play-- or if something is starkly off-- I'll gently ask if they want to expand the scope of what we've done.
I'll talk to people, OOC. We can talk about our RP, our plans for where our stories might go. We aren't scripting things out -- what happens is not pre-determined but we can decide my character wants to go from being more timid to getting into a more heroic context. We can create situations to explore that.
My characters have a Voice, ideally, and I can hear it in my head almost -- and I stay true to that Voice. Who the character is. BUT. I still am the author of that characters story: and I am not an author in isolation.
People I would like to think of as at least friendly if not friends are also authors in the same space of me. Us all having fun and having stories we enjoy matters.
The specifics... vary. Some characters I make are irreverent and though maybe shine from time to time are often at least on some level comedic relief. Others are dark and brooding and trying to find humanity. Others are ... etc. It varies. In each I try to find a person or three who while they are not on the same path as the character in question, are on a path to support/develop/reveal the path itself.
Ideally, when my character grows, someone elses does, too. Ideally, we both end things feeling a sense of fulfillment as a chapter or story ends -- and with mushing, the ending of one step may be fuzzy (it may 'end' for different people in different ways and at different places) and it usually just means a turning of the page to a new chapter or story.
There's lots of different ways to have fun. I know some people who want to have fun by minimizing OOC contact almost entirely-- the idea of talking about characters and future development is anathema. I respect that but its not for me.
I enjoy the game when there is an OOC cooperation (with a healthy level of communication that does not lead into the way of coordination) coupled with an IC world that makes sense. I don't ever have my characters do anything that is not IC for them, I believe in staying true to a characters voice, but there's a lot more leeway in this then some people feel, for me. If a character isn't in a fun place, I arrange situations around the character so their IC choices lead them to a place that is fun. Or I stop playing them.