I don't know why, exactly, but I tend to be more creative when I'm writing up a cast RP profile, where I have to work within definite canon limitations, than when I'm writing up an OC, and have none.
Well, it's much easier to colour than to draw, is it not? It doesn't mean it's any less creative; it's just that if you have a basis, you can use that as a springboard for creativity.