I know exactly what you mean about the shower thing. For some reason, inspiration always delivers it's best lightning bolts to me in the shower as well. I even bought soap crayons so that I could write down dialogue because sometimes I lose the thread before I can dry off and find pen and paper.
As far as planning my stories, it depends. A lot of my stories begin as scenes that simply beg to be written and have grown into full blown, complete stories after some plotting and writing. I rarely sit down with a fully formed plot line in my head but I have a loose idea of where I'm going. Characters development is not something that I plot out on paper. I do give it a lot of thought because I want my characters to be both real and consistent throughout the story. And finally dialogue is something that seems to write itself with me. It flows freely once I have a good handle on my characters.