For me, its part having fun, mostly being creative, and also a third option, of helping my writing with the story involving my ORPC.

I find if I RP with the character, then I start thinking about all the things I need to figure out in my own head about him. Back story, motivations, desires, etc. So as I'm RPing as him, I'll think "In this situation, he wants XYZ to happen for ABC reasons..." and then it will cascade into other situations I'm a little stuck writing on and what he'd be likely to do and why.