So knowing what genres are easy for me to write in is helpful when choosing what genre to write, but sometimes I just get an idea for a character and run with it. It might just be a line of dialogue that pops into my head, but from that I can carry on the character's train of thought and see where it leads. If they start talking about taking the kids to school it might be a more mainstream story, but if they start asking whether their space ship is ready for launch yet then the chances are it's something more sci-fi - or the character is a child playing let's pretend.
I don't like giving advice because I'm not sure if what works for me would work for anyone else, but if I had to offer some advice on choosing a genre to write in, I would say just see where your imagination takes you. If something catches your imagination, the chances are it will be interesting for you to write and you'll have some fun while you create the characters and the situations.
I admit I pinched this idea from Samantha Anderson's post for today where she talked about not getting stuck into one fixed genre for all your writing.
In response, I wrote: I suppose the main reason authors end up sticking with one genre is that that’s what the publishers know people will pay for by that author, although JK Rowling doesn’t seem to have done too badly out of writing an adult fiction book.
Now that's fair enough for people who are paid to write, but what about people who write for fun, or without having being previously published? How do you decide what genre to write in?