The slasher movie is, by far, one of the subgenres of horror that most sticks to a formula. And here is the formula:
Pre-Credits Kill+Character Introduction+Cat Scare*+Minor Character Killed Off+Pointless Drama/Comedic Scene+Secondary Character Killed+Hero(ine) and Killer Meet-Up+Hero(ine) Triumphs+One Last Scare=Slasher Movie
This is, for the most part, how every slasher movie plays out. You have the pre-credit sequence kill, which is either part of the back story or is set in modern day and sets off the chain of events. This is where you’ll see characters defiling graves or having sex when they should have been paying attention or telling stories around a campfire about the killer. If this is a sequel, this is where you’ll most likely see a character from the previous movie get killed off (see Friday the 13th Part 2 or Scream 3).
Then comes the cast introduction. During this point you’ll see a barrage of cliches come at you. Don’t worry! Most will be dead in 90 minutes. This is also the point where you’ll meet an ancillary character. Now, the ancillary character can fulfill numerous roles. They’re the Red Herring: “Who’s that?” “Oh that’s crazy Bob, he lives in the woods where we’ll be camping!” The Red Herring will show up lurking, here and there, through-out the movie and then will end up dead at the three-quarter mark.
There’s the Small Town Sheriff. He will say, in one form or another, “Those damn kids!” before the movie is over. Though he’s going to be an asshole throughout the entire movie, he’ll most likely show up toward the middle or end and seem like he’s going to do something to affect the outcome and give the audience false hope. He’s actually going to be murder fodder and everyone’s hopes are dashed.
There’s the Doomsayer. He (or she) is an old and crusty oldtimer who knows more than everyone else, but will be completely dismissed as being either old, crazy or both. The Doomsayer can also play the part of The Red Herring. It’s a toss-up to whether the Doomsayer will show up beyond the Introduction.
Then there’s the Cat Scare. The Cat Scare is when a character hears a noise, goes to investigate and finds a cat. It is almost ALWAYS a cat. And it’s always a cat that has somehow ended up in a cupboard. I have owned numerous cats, but they rarely ended up in cupboards.
Right after the cat scare, Minor Character death. The Doomsayer is a good choice for this, but sometimes it’s the gas station attendant or the lonely hitchhiker or any person who is not one of the fresh young teens.
Then you have the pointless drama and light-hearted comedy to trick you into thinking that that this movie is more than nubile young people being offed with chainsaws.
This is when the secondary characters start dying, one by one and, depending on how many characters there are, depends on how long this process will take.
After all the non-essential personnel are removed, the hero or, more frequently, the heroine meets up with the monster. If the monster is masked, this is where he’ll be de-masked. If the killer is actually the boyfriend, long lost-brother or the mother of a deformed little boy who drowned in the lake, this is where the shocking twist is revealed.
After the Killer is dispatched, the Hero(ine) and her/his Boyfriend/Girlfriend walk away from the body. Then the body moves, or the little boy comes out of the lake or the second killer steps out of the shadows or the Hero(ine) turns around with a crazy look in her eyes and you know SHE’S the killer now. This is the Final Scare. It can be either followed with a re-assuring shot of the Hero(ine) waking up or a freeze-frame of the Final Scare.
Bam. You don’t ever need to watch a slasher movie ever again. Because you just did. All of them.
-D-
dylancharles.net 
So true.