How do Authors do it?

I have many times wondered as I devour book after book how an author comes up with the story, plot, characters, and dialogue that make up a novel. The thought soon passes as I pick up my next read, but recently it has lingered. I have been pondering it for days now. Do authors know what they will write when they open a new document file and begin to type? Do they create a character, build them up and tell their story as if that character is whispering it to them in their ear? Do they suddenly stop what ever they are doing and say " I will write a book about a Man who marries a woman they live quietly until suddenly he finds out she has a secret life and she finds out her best friend is trying to break up their marriage, suddenly they're thrust into a dark world where she must save powerful people and her husband finds out just how dangerous her secret life is.. will their marriage survive the discovery?.. and GO!!"? Could it be a bit of all the above? It amazes me how they can create so many plots and story lines that keep us so enthralled. I imagine it takes several qualities to be a writer. I believe you would need to have a great imagination and of course language and writing skills. I have the imagination covered but my writing skills are weak. I have never managed to write dialogue successfully, and I am not sure I could put the story I am imagining in my head in words. That takes a great skill that I don't think I possess. Maybe one day I will figure it out but it hasn't happened yet! If you do write let me know what your writing process is, I'm very curious as to how you go about it. So until next time..