Film Genres

The most important thing that the opening of a film must do is to establish a tone so that the audience fall into the same mood as the film they are watching. It doesn’t have to be a big budget explosive opening in order to grab the audience’s attention and sometimes an over the top opening like this risks upending the structure of the film with a sense of where do we go from here? Most films nail this concept, but some just dont make the mark.
Usually the most successful film openings bring a well developed sense of intriguement that will leave you settled into your seat desperate to know more about the story and the characters that are going to be in the film. If it is particularly inventive there will be some dramatic foreshadowing that will enable to hint you at the problems that are going to occur throughout the story.
Horror films
Sound is one of the most important parts to a horror film. It accompanies the images on the screen, but it also helps to set the mood. There is no point having a scene which is supposed to build suspence, and have you on the edge of your seat, when there is an upbeat soundtrack playing! Music in film can set any sort of mood, but horrors it is used primarily to set the mood, and to build up more and more suspense. Arguably the most important part in an opening to a horror film is the music. It immediatly drags you into the atmosphere of the film, and helps set the scene.
Sci Fi
CGI may be more advanced these days but it has become far less imaginative. When it first came out, CGI was used to create awe inspiring images in star wars, and it allowed directors to be much more inventive with the scope of their films. CGI in Sci Fi's are now almost entirely based on CGI, because it allows the directors to open up such a wide variety of oppertunities. Sadly though, there are not many Sci Fi films in production. However the opening to all Sci fi films immediately  show that the 'world' as the characters know it is far in advance of anything we as an audience can imagine. For example the opening to Star wars  'A New Hope' straight away jumps you into a space battle between an enormous Star destroyer and a small frigate. The awe inspiring effects, and the fact it is in space tells the audience that it is a Sci Fi film.
Action
The opening to most action films jumps you straight into the action. James Bond Quantum of Solace jumps you straight into a car chase. The Matrix starts with the attempted capture of Trinity. All these films start with a bang, and it really gets the audience excited, and in eager anticipation of what is to come. Action films need to constantly keep the audience going and in the mood for the next fight, or car chase, or gun battle ect. The tension built up needs to be high, because if there is a romantic bit in the film, and all of a sudden it just jumps straight into a car scene, its not going to be as well enjoyed as if there has been a long build up.