I'm a prospective writer who needs help/advice

My advice is to not focus on RPing at all, because it's totally different from writing a whole story, and to think more about books you've read.

But that's a lazy answer, so I'll do my best to answer your question more directly:

I'm not an RP veteran, but as I understand it you often have to think about how your character will react to other characters and events in the story 'in character.' When writing a whole story, it is absolutely essential to consider this for all of the important characters. Who are your characters? What motivates them? How do they play off each other?

Just as important is to consider the events in your plot: Why do they happen? How are they influenced by the characters? Why should the reader care? The logical connections between your characters, their actions/interactions, and the world around them are a big part of how you keep the reader engaged.

And most important of all: just dive in. The community here's all about helping each other become better writers, so don't be afraid to get your work out in the open.

With writing as compared to RPing, you have much more control over everything. All the characters are ones that you "play", and all the plot lines are ones that you come up with. Like icomeanon said, you have to keep all these connections in mind.

I guess then that I can say that the main difference between RPing and writing a story is that you have more control over what happens. You can get a feel for what every character will do, you can direct the story, and you have some idea where they're going to end up. It's all in your hands.

Since I've only RPed twice several years ago, I'm not experienced with that many differences between RP and writing stories. So if there's a specific thing that you're worried about with your writing, feel free to ask. Again, as icomeanon said, we're here to help.