For me, the characters have to have a personality. I feel that in many horror movies, the characters are nothing more than set pieces being kept around so they can have a cool death scene. There are so many horror movies where I couldn't even name one of the characters which isn't good. We don't need to know every single thing they've ever said, thought, or done since they were born, but they should at least have a personality. If the people are lifeless I won't care what happens to them.
On the other hand, many characters DO have a personality but it's an annoying, unbearable one. That especially happens in found footage movies. So figure out a way to give your characters a unique personality but make them likable and not irritating.

characters are the main and most important thing in any movie if you dont have like able or hate able characters the movie is lifeless.
I want to feel for a character so when they die it means something, i want to root for the good guys and detest the bad guys
thats why in new movies especially remakes like the evil dead are terrible because theres no character development so when people get slaughtered its like who fn cares and just becomes generic and pointless

I think genre matters when you're considering how complex your characters should be. 3-dimensional, fleshed-out and realized characters are usually fine in all cases. Below that, it gets muddier.
Horror suffers from 2-dimensional characters, while 1-dimensional characters are fine. All that's required out of a horror character is that they're either scary (for the villain), or able to be sympathized with as they're being scared (for the victim). Therefore, even having next to no personality or backstory is fine, so long as the actor can play scared or intimidating.
However, when a character falls into something like one of the classic "slasher victim" archetypes, the familiarity (and in some cases, actual revulsion against certain stereotypical traits) can rip the audience right out of the action very easily.
Action movies, conversely, should avoid 1-dimensional characters, while 2-dimensional characters can be great. The main thing writers should avoid when making action movie characters is making them boring. Everyone in an action film should ideally have a
gimmick
, at least, and the audience's enjoyment can be mined from how well that gimmick is implemented.

I agree to an extent, it wouldn't make the villain more sympathetic though IMO. Even though I thought it was easy to root for the Blair Witch as those 3 adventurers were annoying as fck.
Still it depends on how interesting you think the back story is.
No sense of humor-bad movie taste-female avatar-child at heart-hated by nolife troll and PROUD of it

I don't think there's one answer to that. Maybe a back story helps. Michael Myers, Freddy and the Blair Witch become more interesting when you hear their back stories