The reason that I chose this film for my horror trailer is because this is the first horror film I went to see with high expectations of being totally and utterly freaked out because of all the hype that surrounded it.
The trailer isn't like most film trailers as it shows filming within the cinemas of the first viewings of the film. With infra-red cameras you're able to see the reactions of the audience themselves in relation to the film, how jumpy they are and huddled together etc. This gives a sense of what is in store for you when you watch it.
The quotes 'one of the scariest films of all time' and 'genuinely horrifying' from magazines help you guage what you're in for, also commenting on audience reaction makes you pay more attention to not just the film aspect but the audience as well.