I'll break down the usual conventions of a horror:
- A group friends/family
- One mysterious person
- Night time
- One hero/heroine
- Exaggerated weaponry
These are all familiar aspects you expect to find within a horror. People often confuse thrillers with horrors. The difference would be that horrors focus around the gore of a murder, rather than the motive. It often seems mindless and involves very weak characters in the storyline (usually loose women and macho men). Thrillers are a little more complex, focusing around the storyline of the murder, revealing it slowly and often having very creepy, long drawn out scenes. If you changed one of these conventions in the horror, say change night to day, that's when they start become a little more original. People can almost distance themselves from the storyline at night because the activities they do mostly consist of during the day. If you took a horror and made it during the day, it becomes a lot more disturbing. Funny Games (dir. Michael Haneke, 1997) is a good example of the reversal. All the murders occur during the day. It just doesn't seem right and becomes a lot more intense and real. Typical horror movies need these conventions to live up to a horror fan's expectations and keep this surreal, predictable world they've created.
Now lets take the codes. These are aspects of the film that provide the storyline to drive the plot along. It's quite hard to explain, but by taking a scene from Donkey Punch (dir. Oliver Blackburn, 2008) I can almost describe it to you. The group of young, promiscuous 20-somethings are sitting on the boat, sniffing drugs and talking about sex positions. In pops the term Donkey Punch (if you don't know what it means, Urban Dictionary it) which is an obvious code as this is what the film is titled. The boat provides the isolated location and the drugs provide the women to be a little more up for things, shall we say, and a little more unaware. Later on, these factors are a predominant part of the story so they need to be introduced. By having these features play a part in the story so early on means that the audience already have it in the back of their heads, meanings the script writers can include a hell of a lot more murders without having the explain a reason behind them. You're also given the characteristic of each of the characters early on so you can predict who's going to die. E.g. The female that's most up for sex will die first/The one that's the weakest will survive.
This is the case for most genres. They have a reputation and play on the codes and conventions because this is what the audience have become accustomed to. Notice it the next time you watch a typical genre like a rom/com or a horror. They devices are provided right at the very beginning all for you to have a satisfying outcome.