So how does it differ, then, from just writing a fiction?
Well, when an author writes a fiction, (s)he is the god of the universe that (s)he creates. The author gets to control all the characters, all their actions, thoughts, feelings and more. The author gets to say what when where why and who, and (s)he knows, usually, exactly what is going to happen next.
With an RP, however, you only get to create half the universe. Or less if there are more than two players. You create your own character and his/her back story and bio. You decide his personality and you ‘become’ him for the sake of the RP. And you control him, his reactions and feelings and you control his environment as far as his influence allows.
In this respect, written RP is much more like acting. It’s called Role PLAY, after all. Like an onstage play. The stage is the forum (or messenger or email) that you create your universe on together with your partner. The play or story is the plot line that the two of you might have discussed before. The style is more likely than not adlib.
Like actors on stage without a script, told to assume characters. The actors must then ad-lib or improvise the scene, they make it up as they go along. RP is much like adlib acting in that, given a set of circumstances and a starting point, the two actors (or players or partners) begin to act (or type) out the scene or story. One actor will not know how the other will react, what he’ll say or do next, because there is no script. Merely a starting point, and sometimes a goal. Each actor must assume the role and react to their partners accordingly.
It is the same with RP.
In this way the players pass the story from one to the other. Each reacting, responding, and furthering the scene to some extent before passing it back to their partner.
And that’s the fun part!
Because you often won’t know exactly how a partner might respond. Sure, sometimes the plot is discussed in general, and the end goal is often determined. But during each small scene there are often lovely little surprises, twists, turns, etc that will leave you intrigued. It’s exciting when one doesn’t know what will happen next!
P.S. There is a type of fiction called a Round Robin, where a story is passed between authors. This is not the same as RP in that each author still has control of the whole universe. In a round robin fiction, each author adds a chapter (or several chapters or sections) and passes it on. In an RP, it is much more like a few paragraphs on average, controlling only your own characters, before passing it back.