I'm sorry but I disagree. If you don't know whodunnit before you begin writing a mystery then it's going to be a mediocre read. If you don't know how your characters are going to develop and why then again, mediocre. That is unless you're a brilliant writer who can just spew out well-crafted stories. They are few and far between.

I'm not saying you can't write an interesting story doing it organically. I'm saying you'll never write anything great unless you learn all your craft. Which includes coming up with a plot, outlines, and characters before you begin. If a writer can't seem to do this than they need to work on their skills.