What are the functions of a film opening sequence?

The main function of a film opening sequence is to set the genre for the film. We are able to tell what kind of film we are about to watch in the first few minutes of viewing it. For example, we know that Juno is a comedy kind of film because the opening sequence is fun, bright and colourful. We can also tell that Batman: The Dark Knight Rises is a crime/action/thriller film because we see violence, action and deaths from the offset which fits into these genres perfectly. In these two pictures we are able to see the difference between a thriller/horror and a comedy/indie film opening. We can tell the genre by the colour, brightness and music in the opening sequences. 

Another function of a film opening sequence is to introduce the protagonist/antagonist. There is usually emphasis on the main characters in the opening so we can distinguish who's who quite easily. An example of this is the opening scene of The Hunger Games which shows us who the protagonist in the film is and we hear her some of her back story. It also sets the genre and story.

Yet another function of an opening scene could be to set the story/narrative or give it a back story. This is essential as an opening sequence which doesn't engage the audience or link to the rest of the film will have a negative effect. An example of an opening sequence which sets the story is Up which introduces us to the main character and tells us what had happened to him before the story takes place.

These films also show the other functions of opening scenes. They set the time/place for the film and give us an idea of the setting. Some of them use their opening sequence to foreshadow what will happen in the movie whereas some have simpler functions such as to show genre.