There are only two film genres whose success you can gauge from the immediate, audible reaction of the audience. A good horror movie can make its audience jump and scream, while a good comedy makes them laugh out loud. If a joke lands and you can hear a pin drop in the theater, you failed. If the monster jumps out and everybody is still in their seats, you failed. When these genres are so difficult to nail on their own, it's amazing that anybody ever makes them work in tandem.
In the history of film there have been some amazing horror-comedies, from Evil Dead II to Cabin in the Woods, with the odd An American Werewolf in London and Shaun of the Dead in between. But for every brilliant film like those, you get seven or eight variations of Bachelor Party in the Bungalow of the Damned. Why does it seem like it's so hard to get these things right?
1.) Tonal Whiplash
This is the biggie. Laughter is the natural enemy of fear. You can't be scared when you're laughing, and you can't laugh when you're scared. The two emotions don't go together, which makes a horror comedy an especially tricky balancing act. The best horror comedy movies can employ the most enticing elements of both genres without letting them cancel out each other.
A good horror comedy director needs to know when to go for the scare without undercutting it with a laugh, as well as when to kill the ambient drone and erratic violin and let the jokes land. If you don't give each moment its own time to shine with ample breathing room, you risk exhausting your audience and ensuring that they react to nothing. Which leads us to number 2.
2.) Timing Is Everything
You've all seen bad timing in film before. A jump scare doesn't have sufficient build up and it feels cheap and unearned. A scene cuts too soon after a joke and you don't have time to take in the punchline and laugh. In both horror and comedy, timing is everything. This means one of the hardest parts of making a horror comedy is giving both elements the amount of space they need to operate most effectively.
There should be a clear point of demarcation between your laughs and your scares, but it's all about balance. A scary sequence needs to be intense and played straight, but if you stay in that zone for too long an audience will be resistant to laugh when the comedy returns. The very best directors in the genre can make the turn on a dime, delivering tension, dread, and chuckles all within seconds of each other, but this is a rare skill indeed.
3.) Character Is Key
Horror lives or dies by character. Even if you don't do anything particularly new or innovative with your film, if you get your audience to invest in your characters they will be affected by your scares. One of the keys to any good horror film is rich central characters who you can actually root for.
Comedy almost occupies the opposite realm. Most comedies, especially studio comedies, paint their characters with a broad brush, distilling them down to basic personality traits. This distance can make it easier for the viewer to laugh at your character, but it's the kiss of death for your horror. Think back to any of the Friday the 13th or Halloween sequels to see what flat, stereotypical characters do for tension. If you want your horror and your comedy to work in concert you need to adopt a more nuanced approach to character building that means we can still invest in the protagonist when the shit hits the fan.
To my mind, these are the three most important reasons horror comedies fail, and the key battles I've been fighting in making The Mennonite of the Living Dead. The simultaneously horrifying and funny part of the experience is that I won't know if I was right until I sit down and watch the film with an audience.