How do trailers help to market a film?

Trailers help to attract an audience to a film as they can play around with elements of the film's genre that are familiar to the audience in order to make the film look unique and enticing, making the audience think that that particular movie is like nothing ever seen before in the genre, having a fresh and unique take on the genre by contradicting common conventions. By doing this the film is showing off its unique selling points (or USP). These can be elements such as a big-name actor playing a lead or important role, an unusual mix or hybrid of genres or state of the art special effects. 

Avatar (2009, Directed by James Cameron) 

The main USP of James Cameron's Avatar is the special effects as the CGI is some of the best ever seen at this point in time as the film has entirely computer generated environments and characters. The trailer shows many scenes of the human using their state of the art technology like giant mechanical suits, and the life-like well animated Na'vis, the race that inhabits the planet Pandora as well as other alien species that inhabit the world.  The main stars of the movie aren't exactly well known but the movie does have Sigourney Weaver (portrays Grace in the film) who was casted as the protagonist of another popular sci-fi movie, Alien (1979). As Avatar is a sci-fi genre film itself, the fact that Weaver has a role in this film means that they are using her as the star appeal as fans of Alien might want to see if her acting is as good as it was in the latter film.  What also sells this film to fans is the fact that Terminator 1 (1984) and 2 (1991), Titanic (1997), and Aliens (1986) were mentioned on the trailer as they are all successful well-known movies that James Cameron also directed, in this case they are not just using star appeal to promote the film but also director appeal. Interestingly, Sigourney Weaver played Ripley in Aliens once more, so this is not the first time Cameron has worked with Weaver on a sci-fi film, so this fact gives fans an even better idea on what type of character she would be playing in Avatar. 

Cowboys & Aliens (2011, Directed by Jon Favreau) 

As the title of the film suggests, the film is about  the battle between cowboys and aliens. This is a hybrid genre movie, combining science fiction and the historical period of the wild west. The genre choice alone is a USP for this film as there has been no other films that combine these two genres in a film on such a big scale as this. This film also relies on star and director appeal in order to get promotion as the protagonists of the film are played by Daniel Craig (famous for playing James Bond), Harrison Ford (famous for being Han Solo in the Star Wars franchise and Indiana Jones in the self titled franchise), and Olivia Wilde (famous for playing Quorra in Tron: Legacy). The trailer states 'From Jon Favreau ... The director of Iron Man' using the director's success at the 2008 film to help make the film appeal to the audience. Though it is not features as heavily as Avatar's trailer, special effects does play a part in the film's USP as there are a number of explosions and alien mechanics showcased to show how action packed the film is and how realistic the CGI is. 

Battleship (2012, Directed by Peter Berg)

Again like Avatar and Cowboys vs Aliens, Battleship uses star appeal and special effects to draw in an audience. One of the biggest stars featured in this trailer is the singer Rhianna in her acting debut, the trailer makes it seem as if she has a main role in the film. Another famous star that is shown in the trailer is Liam Neeson, famous for his roles in the Taken trilogy. There are many scenes showing the destruction of cities as they are being blown apart, cleverly constructed by highly realistic CGI. There are plenty of explosions to show how action-packed the movie is as well as scenes where characters narrowly make it out of a near-death situation that will keep the audience on the edge of their seats. This film also has an unusual mix of genres as you wouldn't expect aliens and battleships to be in the same movie. The company who made this film use their success of the Transformers film franchise to their advantage and mention this in the trailer as it states 'From Hasbro the company that brought you Transformers'. For the people who enjoyed the action in Transformers, this would be a selling point to them as they would have an idea what the action might be like in this film.