1. Take action that someone can react to. I'm really amazed at how prevalent this is, and all I can think of is that people are afraid to be bold, especially if they are new to RP and don't want to step on people's toes. Back in the day, I used to roleplay on a Lion King forum (ahem), and the few close friends I gathered called this kind of crap "butterfly roleplay", as in: a lion comes into the thread and chases butterflies. Another lion comes in and chases it, too. They get to be friends by bonding over a butterfly. Everyone is happy forever and ever. Look, text roleplay is a training ground for learning how to write and write well. Conflict is your friend, not peace, love, and butterflies. I would not watch a television show about lions chasing butterflies. (I will watch the hell out of a lion framed for murdering his own father!) And I sure as hell wouldn't read a book about lions chasing butterflies. But so often, what I see is people just posting these extremely static reactions. In conversation, they nod their head. They agree. They stand there and do nothing, or offer inane and stupid opinions that have no relevance to what's happening. In combat, they stand there or attack everyone on both sides. If someone else enters the thread and does something, most of the time the new person will be totally ignored. In the thread as a whole, they risk becoming The Load (See Rule #5). In general, these kind of players are are just a drag to have in the roleplay. In real life these people would be considered brain-damaged. If they were a gesture, they'd be a noncommittal shrug. But in RP, they're someone's "character" and that's supposed to make it okay. In RP, I swear I will take a racist, a sexist, and a bigot over the most agreeable person in the world. Because I know who is going to be more interesting to react to. I can have pleasant and normal conversations in real life. I don't want to roleplay something I could be doing in real life, when the point of RP is to ride dragons and shit. In writing, the saying is "Character is action." A character is the action they take. Not what they TALK about taking, or THINK about taking, but what they DO. In roleplay, if your character is REACTIVE, they are not TAKING ACTION. If all your character is doing is reacting to everyone in the thread, and never doing anything themselves, get the hell out of my roleplay that that weak-ass bullshit, because you are not someone I want to play with. Every post you make, you should be providing fodder for your fellow players to react to, in addition to having your character react to theirs.
2. Say yes. This was something Stephen Colbert said about the first rule of improv comedy: say yes, no matter how absurd the suggestions. From an individual point of view, it results in a great skit. In the sense of Colbert's career, it allowed him to do amazing things, just because he never said "No, having a machine on NASA named after me is just ridiculous! Sponsoring a speed skating team is too odd!" Roleplay is a type of improv. And if you think you have to say "no", because it's humiliating, humbling, or not "in character", yes, you've achieved the impossible: you're roleplaying wrong. Way back in the day, I played with a Dungeons and Dragons group, and inevitably someone would play the lone wolf ninja badass, who played by his own rules and was gritty and anti-social. (Barf.) What ended up happening was that the rest of us, as a group, would wait around for hours while the DM ended up running an entirely separate game for Mister Ninja. Eventually, as group, we got tired of that crap. And Mister Ninja waited around for hours while we played our game. And when he complained, the DM said, "Don't like it? Then don't play that kind of character." This gets back to being The Load: it is NOT EVERYONE ELSE'S JOB TO CARRY YOUR CHARACTER THROUGH THE ROLEPLAY. Nope. I don't care. I don't care if your character hates everyone, is blind and mute, and would never, ever in a million years go to a bar to get a drink with folks. Because maybe this time he would, and he needs to, because saying yes is how you will get an interaction. Saying no will get you nothing. "It's my character!" is the rallying cry of the asshole who doesn't want to be considerate of other players. If you don't want to play with other characters, don't roleplay. Go play a video game. The computer won't mind, I promise. Say yes. Does it humiliate your beloved character? Say yes. Does it make the character do something strange or a little odd? Say yes. This rule doesn't mean you have to say yes to extreme things like dying, or something your character rejects with all of his heart and soul, but a drink? Conversation? Something within the realm of a reasonable, sane person? Say yes.
3. Variety is the spice of life. Humans are complex beings, but man, too many of them have a habit of creating one-note character. Like, say, you have an angry character. Kinda fun to interact with, because anger is so volatile. But it doesn't matter what happens, anger is all this guy will ever react with. Insult? Anger. Offer to help? Anger. Adorable puppy? Anger. Who cares after a while? You're writing a one trick pony with one dimension (who probably also lacks in growth, see Rule #4.) Thinking about how your character reacts in different situations is key to making a good character. I recall an old roleplaying buddy of mine who thought that "strong female characters" killed with violent precision on the battlefield and then cried and felt guilty about it later. Which, as a girl myself, made me want to punch him in the face after his umpteenth female character cried about how bad a person she must be for decapitating that ogre. Even just in basic social cues, children bring out different responses than adults. An adult being a jerk is held responsible for their actions; a child being a jerk is either blamed on bad parenting or an off day. Similarly, if you see a kid being called names by a parent bent on shaming and humiliating them, you respond entirely differently than you would to two adults doing the same thing. Maybe you do have an angry character. Are they going to get angry at a disabled person? A mute? A child or someone very old? Different nuances in different situations should bring out nuanced reactions. And sometimes, yes, this means your character steps outside their comfort zone for the sake of interaction. I might play a quiet, thoughtful introvert, but if the rest of the gang is going somewhere, I'm probably going, too. Roleplay lends itself to wonderful variety, because there's such a variety of players, and it's a lot of fun to see your character respond to the unexpected. A recent personal favorite moment in roleplay was when a friend of mine was playing a kind of obnoxious guy on a ferry, while my quiet introvert was a passenger who could see the ferryman and other passengers were really bothered by obnoxious guy. So when the obnoxious guy put his feet up, my introvert tipped him overboard. (My friend had to leave, so it was a perfect way to excuse her from the RP after I asked if I could.) It's become a great moment, because my introvert earned the friendship of the ferryman and the passengers, and that in turn led to further interactions. There's no limit to what can happen if everyone is on board with asking questions and saying yes.
4. Keep growth in mind. I happen to think roleplay, especially text based roleplay, is a wonderful training ground for writers. Mostly because what makes good roleplay makes for good writing. Roleplay with tension and high emotional stakes and a great plot is just as enjoyable as a book with the same elements. But a common problem I see with RP characters is very similar to a lot of protagonists: all their growth happened in their past. Moving forward, they have nothing to gain because they're already who they will be until they die. Which makes for some pretty damn boring interactions. They're never going to change their mind, have sympathy for another point of view, and they are certainly not going to change. The easy answer for this is "trauma." "Yeah, man, my character had really bad, traumatic things happen to her." Well, for the most part, (barring something like wartime PTSD) trauma actually has been proven to lead to self improvement in a vast majority of cases. Seeing or experiencing suffering creates compassion. If you see a bunch of people living without water, you suddenly get a lot more aware that water should maybe be a human right. If you see dogs dying of neglect, you might get more protective of a stray or be motivated to work at an animal shelter. If you suffer from depression or anxiety, you have more sympathy for depressed or anxious people. It's actually if you are super protected in a bubble all your life and never had any difficulties at all that you are more likely to be lacking in empathy for others. Even if growth is small, or even if just one character brings out something different in yours, growth is still important and rewarding. And a necessary part of writing a good character arc for a book. Characters need to be changed by events in a book, so you should have them change in reaction to roleplaying events. An introvert becomes a little more sociable. A loudmouth learns to shut up and listen a little more. Big, sweeping changes? Even better! Go for it. But don't stagnate your character just because "It's my character!" Especially when you're using it as an excuse not to interact with other players.
5. Don't be a Drama Queen, The Load, or a Damsel-in-Distress. Everyone loves drama, especially teens. But if you are any of these things, you are, essentially, placing the burden of action on others while expecting the plot to remain centric to your character. You tailor your actions to limit the actions of others into doing something you want them to do. Everyone centers around your characters desire's, and if they don't, you're not playing at all, or playing so badly you may as well be punishing the other players. For the last time, roleplay teaches you how to structure story and character, and this is about as close as you can get to being a Mary Sue. Everyone else is actually doing stuff, but you're reaping the benefits. For example, your character is crazy. Certifiably so nuts that no sane person would ever get near you, as you smash things, attack guards, and do other wacky hijinks that have nothing to do with the plot. You're the Drama Queen, especially if you whine when others don't want to play with your idiot loser of a character. If you never react to anything, never act out your own ideas for the situation, or give anything for the other players to react to, congratulations, you're The Load. The rest of the characters will tolerate you and haul you around, because let's face it, you're dead weight, but the next time they go hunting for someone to play they're not going to extend the invite to you. And if you get to sit and do nothing while your companions fight and struggle, for a cause that means nothing to them but everything to you: you're the Damsel-in-Distress. Emo characters who constantly have to be stopped from self harm or overdosing or killing themselves are very popular for this. They have to be "rescued" from who they are, and even if a nuclear missile is five seconds from launching, they're about to shoot up that heroin so save them! And don't think these are mutually exclusive terms, either. One of my absolute favorite stories about how NOT TO ROLEPLAY was experienced by a friend of mine on an animal forum years ago. She was a jaguar, and started a thread where she was hit by a tranquilizer dart from a poacher. THREE OTHER PEOPLE came in with their own characters and made the exact same thing happen! The thread died six posts in because all it was was a bunch of unconscious jaguars at the base of a tree, an idea so fabulous in its terribleness I get tears in my eyes laughing about it every time I think about it. How magnificently stupid can you get? It's a brilliant and awful combination of Loads and Damsels-in-Distress ("Hey, sweet! I don't have to contribute my own ideas! I'll just copy the OP and then MY character will get carried around by someone else's plot!") too scared to come up with anything original.