It takes two people to RP, both those people should put some effort into their work, it should be fun for everyone right?! No one is being forced to RP here and no one is being paid to RP here so having good manners is really, really important (we're all friends so please keep all communication friendly and respectful).
The more descriptive both parties can be the more fun the RP will become! Placing your character's speech in quotations ("-") helps us understand what your character is saying, so they're super important, otherwise how can we know what your character is saying?
In order to make a really cool post that impresses everyone, rather than just posting dialogue, you can describe your character's actions before, after, or in between their speech. This can make your character and the RP seem more real.
Another useful tip to add onto this is the way you describe things and how to improve your post length in general. Does your character do an action? If so, how do they do it? Why do they do it? Is there a reason for why they do it a certain way?
Talking about surroundings and accessories can also help. How is your character reacting to where they are? To how the guard is treating them? To how the prisoner is acting? Are cuffs rattling when you walk? Are they doing it naturally or is your character making it happen?
You'll be surprised just at how easy it is to lengthen a post simply by describing things that can be described, though keep things relevant! For example do not describe how things are outside if your character cannot see outside!
It's little things like this that help to improve your skills in text-based role-playing and with time and effort you'll see a massive improvement to yourself.
Some other things to consider are metagaming, powergaming, and god modding. In most game-based RP these are major no-nos. These are just fancy terms for not taking control of another person's character without giving them a chance to react, using information you might know but your character doesn't, and giving you character abilities that they normally wouldn't have. That's about it, I hope you have lots of fun RPing!