There is an opinion among movie lovers that sometimes the trailers tend to give away a little too much of the plot of a film. Used correctly, usually under the direct supervision of the film maker, the trailer can be a tool to tease and create a buzz for a film. Recent examples that come to mind are the Inception and Pacific Rim campaigns. Left in the hands of the publicity team the trailer can sometimes be a too revealing.
An example of the latter is the trailer for the forthcoming drama, Prisoners. Every plot development and twist is telegraphed in the two and a half minute teaser that there almost seems no point in actually watching the film. The talent on show is impressive enough but I doubt that the trailer will go any way to actually spend their hard earned cash going out to see something that has been shown for free online for months before. You can make up your own mind.
Prisoners is out in the cinema this September.