Horror idea #1

I came up with this idea something like a year ago, because horror is the best genre ever.
Most horror movies these days have ominous titles, scary, dark trailers, and generally let everyone know it's going to be a horror beforehand. Therefore, most people who see them expect to be scared, right?
What if a movie was made, where all the ads, trailers, actors, and the title had absolutely nothing to do with horror? Make the trailers to be some romantic comedy. Ads are colorful/whimsical. Have Vince Vaughn or someone to be the main character. Basically, the only thing that could possibly tip anyone off is the 'R' rating on the trailers.
The movie would start off colorful, full of jokes, and the character finds the girl of his dreams. He gets help from his best friend, they fall in love, etc etc. Standard crap.
At some point, start slowly showing odd things. Maybe the relationship hits a snag, and at that point, maybe put something... weird in the background. Put a little less makeup on the lead character/s, dim the lighting. He'd come home or to his friend's house to find his friend dead. Like, eviscerated dead. Blood every, guts, the works.
From this point in the movie, the character is being tormented. The color is bleaker (tones of grey/sickly green). There is absolutely NO music, except maybe at most a couple scare chords. The actor/s will have no makeup on. Maybe random people in the background will generally do impossible things; have an extra limb, sunken eyes, walk behind a pole and don't come out the other side, but never in focus or emphasis. People will bleed from their eyes; commit mass suicide without anyone else noticing even their existence.
As for the plot, the only thing I could think of is some sort of Lovecraftian shtick; some sort of unstoppable omens, a cult attempting to communicate with an unfathomable (and never, ever shown or heard from) dark god.
People would be shocked, disturbed, and probably very pissed at this movie. The main crowd at first would likely be couples or people on dates, expecting to just see some light-hearted comedy. They would be set in that expectation, too. The worst scares are always the unexpected ones, so why not make that the entire point of the movie?