Their is a major difference between the opening sequence of a film and a trailer.
A trailer will be composed of different parts of the film almost all the time the best parts to encouarge the targted audience to go out and see it. It will open up with the people who helped make the film such as Dreamworks or Paramont. The trailer will never give you a huge idea of who the main characters are just what the film is about and what happens in it for example a link of a trailer below is found here.
While the opening sequence to a film will have the Title of it and a short brief and close ups of who the main character is. The producers want you to recognise the main character from the start so to do this most of the time he will be the first person you see. Howver before you see the main character you may be given a establishing shot, this is to show the audience where abouts in the world or what planet the film is set on.
