After visual design and modest biographical detailing, the techniques an MMO roleplayer can use to help express a character, widen the immediate effect of the character's persona in the moment, are dialogue and emotes.
Dialogue could be a blog post in itself. There are entire books written by smarter people than myself about how to create dialogue for characters, and how that dialogue creates the characters themselves, in fiction. My cheat sheet for dialogue is this: Figure out how your character speaks and stick with it. If your character is a formal fellow or well educated, avoid contractions (can't, I'd), if she's female avoid using "I" often and find other ways of framing a line, if you're trying to use an accent trim it down to only some words otherwise you'll be incomprehensible. That last is important and learned from experience. What sounds good in your head sometimes sounds like white noise when a nonpsychic tries to translate it on the screen. Sometimes just mixing up grammar rather than distorting words will do the job. Or, to put it another way: "The job's done, betimes, not wi' words themselfs but wi' where fall th' words do." Yarr.
Emotes come in three flavors in MMOs. Animated, basic and freehand. If dialogue is what your character says the emote is what he does. Animated emotes are gestures or movements or facial expressions that get picked up and displayed by the character model. Basic emotes are simple words or phrases that are expressed as text on the screen. Example, I'd type /bye and the game might say, "Mandash wishes you well and hopes to see you soon" or /smile and you'd get "Mandash smiles."
Often simple emotes and animated emotes work together so that a /bye or a /smile in Star Wars Galaxies would animate the avatar even as it expressed the associated text. Galaxies also, and some like this better than others, will pluck words from dialogue to animate a character. If Grim says "pleasure", as in "the pleasure be mine" out loud he'll bow unless I've disabled that function. In other cases it's more of a problem. If he says, "Late I'll be ter th' rendezvous, I fear" the tough old captain will start shaking like a little child on a cold winter's day with his arms wrapped over his chest. It's fear! *goes to options and disables automatic animations*
Freehand emotes are the most important of all. This is where, along with dialogue, you get to show your roleplaying chops. You're simply writing out an emote. Examples: /em shakes his fist at the heavens. "Damn ye, Star Wars Galaxies, I ain't a'feared o' yer cheesy animations!" Or, in MUSH style, :shakes his fist at the heavens...etc. True MUSH style also includes ";" for possessive poses as in ;'s wrathful voice echoes throughout the blog post. This is painfully lacking in SWG and most other MMOs as well.
The secret to freehand emotes is context. Know where you are, what's going on and try to imagine every little detail about your character and his environment.
We all fall into habits and common expressions and that's actually a good thing to an extent. Players really tend to be most focused on their own characters so repetition of certain key phrases or words that make your character stand out, over time, helps people remember who he is and what he's like. For example Grim will almost always "amble" or "meander" when he's moving. Sometimes he'll "stride" or "stalk" if he's in a black mood or feeling particularly energetic about something. I don't even bother looking for other words. This does the job for that.
But don't, I plead with you, fall into the habit of creating macros for long poses. There are players who delight in creating long "stock" emotes and unleashing them on the unsuspecting public over and over (the public doesn't stay unsuspecting for long). This gets in the way. It's lazy. It's not responsive. And it's often spammy as hell since the player's not having to actually think or write but hit a single key on his keyboard to unleash a torrent of disconnected creative typing. They pour over their favorites like Gollum with his ring. They'll sometimes even warn you ahead of time in an OOC chat or /tell: "Check this out. I just made this one!" This is solipsism not creative, immersive, interaction with other players.
Now when I say to imagine your character and your environment I'm suggesting you really think about him in that moment and what, or who, is around him. The more stuff you can put into poses, over time, that make your character seem distinct while also showing you're aware of what's around the more immersion you're creating not only for yourself but others. In general, don't show it all off in one pose. Keep them short, usually, but with a telling detail here or there. Idly scratch an itch, adjust a gunbelt while eyeing the Dosh by the bar counter. Amble, and/or, meander to a corner table. Grin wolfishly as you slump down onto a creaking chair.
Why keep poses short if more detail means more immersion? There are four reasons. Two are technical while the other two are psychological.
One technical reason is that often you're around a bunch of other characters. Big poses can fill up a bit chunk of the limited space in the dialogue box. You might have just shoved dialogue, or poses, other people are still reading and trying to react to clear off the screen. Yes, they scroll up but that's an immersion breaking pain. This is more true in crowded situations than in smaller interactions but be mindful that you're not the star. You, like everyone else, are a supporting actor. The scene itself is the focus. Don't overwhelm it. Insinuate yourself into the fabric of it.
The other technical concern is the ability to interrupt. If you have a penchant for extended poses or long speeches other players have to sit there passively. They can't really interact or insert their own contrary ideas before your character's finished his monologue and set down Yorick. This busts immersion. Now if your character's an entertainer telling a story or a politician giving a speech, don't sweat it. Otherwise remember your character needs to breath, to pause and ponder, and other characters need an opportunity to interject, to counter or to agree. Besides, long poses take a long time to write. Roleplaying isn't a spectator sport. Few are happy about waiting on someone else and most will simply keep on going unaware of the brilliant storm of rhetoric you're brewing up. By the time you're done the moment may well have already passed.
One psychological reason to stick to shorter poses is that you safely avoid looking like a show-off. A big, fancy, pose filled with allusions and flourishes and so on in the middle of a terse discussion can send the impression that you're trying way too hard and likely looking for attention.
The other psychological reason is that you can overwhelm other players. The goal of detail breeding immersion is to help other players get deeper into their own characters and the moment as much as you into yours. While your pose is entirely an IC, or in-character, act there is an OOC reaction too. You might inadvertantly dominate a dialogue by having your character forcefully, and at length, make a point. Dialogue and posing in roleplaying isn't a game of ping-pong you're trying to win by slamming that ball so fast the other player can't hit it back. You want to measure your responses and sometimes even bunt to give the initiative to your opposite numbers. Quite often less is more. The absolute least, the much maligned "..." is actually a very handy tool. It lets other players know your character is paying attention but is either shocked or at a loss for words without putting too much english on a pose. It gives other players permission to interpret the moment as they want to.
Edit: Related to this last consideration is that posing a character's thoughts is generally seen as poor technique. It's internal, meaning other characters can't react to it, so it's aimed at other players by default. That's bringing OOC into IC for them. If they can't do anything with a pose other than be amused or offended by this as players then you've moved from IC roleplaying to emotional manipulation of another player even if you don't realize it. I know, it tickles some people to pose characters thoughts and witty thoughts can create warm chummies, but it's more effective to have your character accidentally think out loud even if quietly. This gives other players the option to interact with the pose with their characters.
Hopefully this is the discussion Tychus was looking for. If not I'm certain he'll let me know here or elsewhere. But preferably here. Sakky, Scotland Tom, Vox and Neopythia: the courtesy of a response is requested. Same with everyone else!