Now, when you're posting, the first thing on your mind is probably responding to the post before you, right? If so, that's perfectly normal and perfectly acceptable, but when all you're doing is responding, it can get pretty boring. If not for you, than for the person running the RP. So, even if you're post is over 150 words, I advise you to take the initiative. Have your character ask a question, or bring up a different discussion (not in the middle of someone else's, though, unless your character is supposed to be a jackass or something). Think of how you would talk to someone in real life. If you're just replying to what they say all of the time, you're not going to be talking for long, are you? For example, if the answer to "How are you?" is just "Good", then there's the end of that, unless the person has some kind of motive in which case they may ask "Why are you feeling like that?" Very few RP characters are like that, however, just like very few people are like that in real life. The ideal response to "How are you?" should be something like "Good, and you?" This allows for the conversation to continue and gives the character who asks the question something to do. Now, you may actually be antisocial and not care about that in real life, but when you're RPing a social character, you do have to be social.
Now, if your character is antisocial, you can't necessarily act like a social butterfly, and I understand that. And, even if your character is that social butterfly, there may be times where you can't really say much. However, you can always think, I'm sure. You can start with explaining your character's response with their thoughts, either before they do it, after they do it, or even while they're doing it. Then, try having the character think about what other character's responses might be. For some people, this is harder than others. The best way to think like a character is to make sure you know the character, first of all. Read over the character sheet or old posts if you have to. Then, make sure you know what's going on, either by reading a summary of the plot, old posts, or the setting of the world as a whole. Finally, just put yourself into your character's shoes and write down what you would be thinking. Take your character's personality into consideration. For example, if you're RPing as an honor-bound knight and he saw a collapsed maiden in a dark alleyway, if the first thing you think when you put yourself into the knight's shoes is "Check her out, first", then that probably wouldn't make sense for the character. However, it may be interesting to note some kind of small moral conflict in the knight, like a little voice in the back of his mind that tells him to think and act upon such things.
The same goes for battles, as well. You'll not only want to post your character's actions, but also some kind of explanation for them as well, and the character's thoughts as he or she performs them. This makes the post more interesting, and less like something out of a choreographer's notebook, to use a term that I read once.
