In January J.J. Abrams dropped the trailer for a movie that nobody had known was being made. Meredith Woerner wrote about the secrecy this entailed:

• The actors were told about the movie by their agents, but, unusually, the agents weren’t given a chance to read the script. They were merely told that JJ Abrams was involved.
• The scripts themselves self-destructed after the actors read them.
• The movie largely takes place underground, and once the actors arrived on set, they basically weren’t allowed to surface.
• Interestingly this may have helped them better embody their characters. Mary Elizabeth Winstead says that being underground for so long meant that the set eventually began to “bleed into reality”.
• All this secrecy – including a fairly unilluminating trailer – was done so that audiences would be able to watch the film with a fresh perspective without anything being spoiled.

Read more about the movie, and what the actors thought about it here.

Source: Los Angeles Times