I’ve never actually written straight horror, fantasy or science fiction. I exclusively cross genres in my writing. Why? Probably because I love most genres and I love where they intersect. How is it done with horror? Glad I asked for you. Here’s a little bit about the way I see it.
Know the goal of the genres: I’ll just go with speculative genres in this piece. Fantasy inspires awe and wonder through means plausible via the imagination; Science fiction inspires awe and wonder through means plausible via science and imagination (for Hard SciFi, add “stringent adherence” to science); Horror invokes horror, repulsion, dread, etc. But wait. See how one chooses to inspire and the other to invoke? They are completely different goals and that’s what makes Horror a great genre for blending.
Dominant genre: Is there one? You better make that clear. Take the movie ALIEN, always my favorite example of crossbred scifi-horror. Which is the genre then? Science Fiction-dominant, or Horror -dominant? Victims are getting picked off one by one by a monster, leaving behind a strong woman to save the day. Sounds like the plot of many a horror movie. And yet, here you have this species from another planet that gestates within a human body, evolves into a larger creature, and you also have space craft, hypersleep, and androids. SciFi enough for ya? So, again, I ask: which is it?
My answer is going to be qualified, of course. ALIEN is Science Fiction with Horror. I distinguish horror as both a mood and an emotional goal, but there are also tropes that belong to the genre. ALIEN is scary. People are dying one by one. Horror-ness does appear strongly in the film, but the more traditional horror elements like supernatural beings, ghosts, bad places, etc are not in the forefront. Elements of Science Fiction are, and that’s what makes it the dominant genre. Here’s a flipside example. I AM LEGEND is about vampires, and yet the curse, now a disease, is explained through science. You have the main character venturing into dark vampirian lairs to hunt them down and kill them. Life is a constant physical and psychological struggle for survival against these beings. So this is Horror with Science Fiction.
Caveat: For some Hard SciFi purists, there is no sense in calling a story science fiction unless the main problem(s) is identified as scientific in nature. That would make ALIEN only a horror movie, which I disagree with.
How many genres should I blend? As many that makes sense. There are elements of Western in STAR WARS, but that is largely caused by the similarity Space Opera has with the Good Versus Evil storyline. Yes there is a cantina scene, and yes it’s very westernesque, but you can also make a case for samurai movies in the light saber battles and Jedi philosophy. All these elements are quaint little asides to a story that is really a Science Fantasy. My novel DUNGEON BRAIN is what I like to think of as Military Science Fantasy. The science is not dwelled on in a meaningful way, but appropriated with some care to some tenants of science. Moreover, there is nothing supernatural revealed as the cause of the other-worldly phenomenon. For the Military genre, there were war scenes, bunker scenes, and the wide-ranging “war is hell” theme that crops up in the tone.
Once again, these are parts of the whole, and my advice is to take them as they come. Don’t set out to cross as many genres as you can. Let it happen naturally and it will feel better to write, and to read.