Genre, like theme, brings up bad memories of often bad English Lit teachers, who managed to take what should have been a good experience in school and turned it into drudgery.
“What’s this book’s genre?” The teacher would ask you, after forcing you to read something absolutely terrible.
My English lit teacher handed our class 1984, and forced us to read it. He demanded theme and genre.
“Where does this book belong?” He asked. My answer was immediate. “The fireplace.”
It wasn’t the answer he was looking for, but it – was my heartfelt answer. I have to agree with one book editor I heard who described “Catcher in the Rye” as being a dreadful book where nothing happens. And amusingly, there’s even a movie whose premise is that this book exists only so the CIA can track serial killers they’ve trained!
If my English Lit teacher had put the same work into some of the other books he had us read (Like “A Wizard of Earthsea”), things might have turned much different, and we could have been an entire class of professional writers!
Genre is important, like theme. Each genre has rules. If you break those rules, you do so with impunity – because your readers may abandon your book if you do.
Make sure that you stick to the conventions of your genre. If it’s a whodunnit, you have to decide if its noir, cozy, armchair, or gritty realism. Don’t violate the rules of the cozy by having everyone in town leftover cast members of a Stephen King novel. And especially don’t violate the rules of the cozy by featuring a realistic, horrific death by stabbing where blood flows and people are screaming, and body parts litter the floor. If that’s what you want, then make it a gritty realism story, with its conventions.
Theme must be bundled next into the genre. If you’re writing a sci-fi and its about distrust between an alien species and humans, this then is your theme. Every scene in the novel must advance that theme and to a greater or lesser degree that plot.
Can you combine two or more Genres? Yes, BUT ONLY IF you make that clear from the first scene and paragraph! A Noir mystery sci-fi steampunk novel can be written – but pitch that right from sentence one.
“Life is hard on Deneb IV, what with the twenty-odd hours a day of dim twilight, punctuated with the blackness of night. That’s where I found myself when my brass time machine dumped me in the alleys of alien ruins, looking for a killer as ancient as he was ruthless.”
Clean that up of course, and tighten it- but that’s how you combine genres. you can just hear the saxophone playing in the background, as we see the refugees from Star Wars’ cantina scene milling about – and one wearing a waist coat with pocket watch checking the time as the hero walks past.
Genre has rules. You cannot violate them any more than the rules of nature once your story has commenced.