Roleplaying is at its best a collaborative shared storytelling where players invest time and creative energy into developing a character, and then sharing that with others. It typically works better with familiars, so that ongoing stories/connections can form and grow. The following is just my opinion, not gospel. Also: rambles.
First thing, spend time on your character concept. Ask yourself, "what would be an experience that I, as a player, would enjoy, and could sustain for hours and hours with others?" -- I've found that this comes down to 'archetypes'. Not the Combat/Powerset meaning of the word, but "what kind of character" (e.g. "The angsty loner"). In other words, personality and premise of the character. I would argue that the dialogue--their words--and the situations a character finds itself in as a result of that personality--is the true essence of a character. It's why powers don't *truly* matter, and why, I believe, powers are more like a device or metaphor. I would advise anyone coming up with a new character concept to consider what sort of actual social and plot-driven experiences they'd like--before picking 'powers', or even appearance.
After you nail down archetype, personality, looks, and abilities/skills... you write. Answer the following questions: What will this character be DOING? What kinds of stories will I be able to write and share with others? Where did they come from? What's their background and origin? What do they love? What are their flaws?
Another good idea is to do a few 'test runs', practice sessions, so that you can get a little head start on finding a character's "voice" for dialogue & interacting with other players. The best way to do this is to write dozens of paragraphs, short story style, where you tell a tale of moments in the character's life. If that seems too solitary, I recommend throwing the character into a place with 'randoms' (Like a public gathering place) and talking up total randoms just to get your feet wet. Doing this will have you more familiar with the character so that when you do get the opportunity to engage in tangible plots & storylines with other players, you'll have the basics of your character figured out.
BUT: to talk about the Roleplay Experience In General, here's some thoughts I have that seem important.
-- Immersion is important for creative and more fulfilling experiences. Like an actor playing a part, we do better when we really try to imagine the experience, and the emotions connected with them, and try to experience them ourselves...at least a little bit.
-- Roleplaying, at its core is IMPROVISATION. If you are the worst at this, don't bother RPing. You don't have to, necessarily, spit out ingenious new story lines on the turn of a dime, but you *do* have to, at the very least, be willing to work and adapt to what other characters in your environment do. A creative spirit--and a willingness to be flexible and change--really help. Roleplaying is a WRITER'S sport.
-- IC is not OOC! This, to me, is the most important lesson for having a sustained, positive experience. In COH, traditionally, Out of Character (OOC) chatter is done with double-parenthesis ((that look just like this)). In other words ((if it's like this)), that means it's the player speaking and NOT the character. Likewise, no matter what a character does, however asinine, annoying, or boring--other characters should interact with them as characters. This also means that characters *do not know* what their player knows... at all. Most importantly, it means that if a CHARACTER is a jerk, it's inappropriate for another PLAYER to have deep feels about it. Characters interact with characters, and the "meta" level of players is kept on a different channel of communication. Don't mix the streams, Egon.
-- Bring something to the table, and let others do the same! RP is @$#@ pointless if everyone is standing around with no ideas of what to do, say, or experience. Come to sessions with a few ideas, even if they're just little ones... like jokes or feelings. Don't be a wallflower. The other extreme is even worse, where some players absolutely assume that their stories are always more important than other players stories--and constantly take over every gaming session. This is meant to a collaborative experience... to do that, there must be balance. A byproduct of the extreme self-centeredness is what people refer to as being a "munchkin"--which are both essentially treating RP like its competitive. It's not. In RP, *every player* is supposed to "win" by having an interesting creative collaboration session with others, i.e. Telling Good Stories Together. This also means that no character should be perfect, have all the solutions for all the problems, etc--this kind of character is called a "Mary Sue"--and they're considered hack. Lacking depth.
-- Consent matters. I myself have bent these rules in small ways in my RP life, but as a general operational rule of thumb, anything that might affect another character, it's considered appropriate to ask the player. For example, you can attempt to shoot a gun at another player...but it's up to the other player if they get hit by it. Even better would be to ask the player before even starting the action. Consent is mature behavior, and avoids a whole host of drama.
Anyone else want to chime in?
-Tipsycakes