I usually just draw without any idea in mind, and sometimes I create a character accidentally, or I do studies and the same thing happens since I'm imagining the person behind the pose and stuff.
Kinda creepy but I have full blown conversations while drawing characters, I'm in character and my brain throws them into a situation and I start having a debate with another character. I guess it's weird but at some point I won't even really know what I'm talking about anymore, it's just happening ^^'.
And sometimes I just have a random thought that then developes to a scene. Then it escalates and whoops I have a whole story to tell. That's kinda what happend with my webcomic. This idea didn't leave me alone, so I sat down to draw it and while drawing it it really blew up and morphed into a whole universe.