Oh, and preparation is a thing too! Before you RP, it's always best to discuss what sort of stuff you're into, agree to a story, compromise on the setting, that sort of thing. This is where'd you'd set limits as well or requirements, like whether or not you want it to get sexual or if you want to inflate or not.
Forgot to mention too, usually brackets are used to talk outside the RP. Well, this is instant/private messaging RP I'm talking about, it's different on actual open RP forums.
Ex. Person 1: (Hey, can we stop for five minutes? My bladder's about to explode.)
Person 2: (Lol sure.)
This is the same for stopping too if you have to go, could be any message really.
Oh yeh, and stick to good vocabulary and punctuation! I can already see you type lovely, so that's a head's start. And try only to respond once, it's okay if you forget something to add something in quickly, but your partner might have started typing already and then it'll be a mess when they see your second response, try to respond to that, while you start responding to their last response as well.
Well, there's a good few ways to RP. It depends on your preferred style I guess, including the length of posts and structure, if you could call it that. Usually, if you were starting, you would write a small summary setting the scene, maybe even describe your appearance as well.
Ex.: I cautiously enter the creaky door to the abandoned warehouse, taking a few steps inside and then giving a small twirl to get a 360 degrees view of the place. The place is huge, open and dark, with numerous machinery scattered about. My bright green t-shirt and blue jeans stand out against the darkness, as does my pale face. I give my thick brown hair a good scratch, giving the place another look around with my big blue eyes, before shrugging. "Okay, it's clear, you can come in!" I yell back at you.
Then after that, your partner, or you if it's your partner who's started first, would respond in a similar way but keep to continuity, like respond to your or their actions.
Ex.: "Okay, I'm coming in!"... Etc.
What you should be careful of doing though is power-RPing. Basically this is when you force things upon your partner without giving them a chance to respond properly, you can't just shove a hose into someone's mouth, you have to instead allude to it, in the act.
Ex.: Person 1: With your back turned to me, I snicker wickedly as I encroach upon you with the big thick yellow hose behind my back and position myself directly behind you, readying the hose above my head and hovering my other hand over your shoulder... I lick my lips in anticipation before poking your shoulder.
Person 2: Shocked, I turn around on the spot with my mouth open wide, gasping in fright... totally caught off guard. I spot the hose and stretch my hands out, but I'm too late.
Person 1: "Say aaaah!" I tease playfully before grabbing your shoulder tightly with my free hand and forcefully shoving the big thick yellow hose between your puffy lips, deep enough that it may be hard to get out. I giggle at your predicament and already picture you beginning to bloat up.
I hope you get the idea (I want to find out what happens next), that's usually the most popular method but there are others with less or more narrative (ie. a quick bracketed action to fully detailed paragraphs). The length depends on the response I guess but some people don't like short responses whereas others will hate huge blocks of text.
Time's a factor too, could keep an RP going for weeks if you write a long response every night, some like that but I've tried it and does get really longwinded. Other RPs would have instant replies, the standard and preferable method, since it keeps you engaged... and obviously because you want to find out if that chick is going to inflate or not. Short responses are then obviously preferred.
Up to you which method you prefer and I'm sure loads of people around the site would be glad to be your test subject, including me! Just give any of us a shout.