- Inform and intrigue.
- Feature key plot lines.
- Appeal to target audience.
- Quick cuts between shots to sustain interest.
- Build suspense/storyline with trailer left on a cliffhanger.
- Introduce other themes to attempt to capture extra audiences.
The most important thing for a trailer to do is to inform the audience about the upcoming film, and to intrigue the audience to compel them to see the film. This can be done by featuring the film's key plot lines in a way which appeal to the target audience; specifically showing the scenes which will be the most impressive and/or intriguing.
Also, cutting quickly between the scenes in the trailer ensures that not too much of them is given away, which would spoil the film, and also that the audience, who would have a lower attention span when passively watching a trailer as opposed to when focussing on a film, will retain interest as new scenes and plots are revealed to them in a short space of time.
Building a storyline with a trailer in a suspenseful manner will intrigue the audience, and then leaving it on a cliffhanger will compel them to see the film as they wonder how what they have seen will come together and conclude.
Introducing other themes (e.g. the love life of the main character) can help to appeal the film to extra audiences outside of the target audience, and assure that the film won't be targeting too small a niche in the market.