Purpose of opening

The purpose of an opening in a thriller film is to engage the audience and to keep them excited for them to watch the rest of the film. You want a good opening to keep the audience interested and you also want to give them a slight bit of information for them to be interested with and to give them a taste of what the film will be about and what will be in it. However, you don't want to give them too much information in the opening so that you would give it away as if you do they will know it all and won't want to watch the rest as they will already know it all.