There are two main types of voiceover within trailers....
1. A narrator describing the film - someone who is not in the film themselves but personally I have never seen this in a horror trailer. An example is here...
2. The characters are talking over scenes of them doing something else.
Mine is going to be more like number 2- my protagonist, Lauren will be talking at the start about the events at the start of the film and the audience will think it's a narration but towards the end it goes onto the scene where she's actually talking to her friend on Skype about everything that's going on.
I've come up with a script below:
Lauren: (VOICEOVER) When we moved here everything was going so well, the house is so nice and so are the people here.
***
Lauren: (DIGETIC) To her friend via video. Hi, we're really missing you but I'm gonna give you a tour of the new house!
Freddie:BOO!
Lauren: UGH! Stop trying to scare me you idiot!
***
Lauren: (DIGETIC)So here's my bedroom, we're not allowed to paint yet because it's only just been rebuilt after the fire but just ignore that.
***
Lauren: (VOICEOVER) We were so happy there in this big, beautiful house but then things started to go wrong, and I think it's because(DIGETIC) There's something wrong with Freddie
Tasha: What?!
Lauren: I, I don't know(VOICEOVER) he looks really unwell and weird things have been happening to me
Tasha: Like what?
Lauren: Well first there were strange noises and flickering lights
Tasha: Maybe it was just a powercut?
Lauren: No, no it was more than that. I'm hearing footsteps in the middle of the night, loud enough to wake me up and when I do, he's there .
Tasha: Well what are you gonna do?