I think that if the director and the production staff wants this movie to stay a secret, then they should respect this if they want to continue to stay on the cast of the film. It would be really messed up if someone was given an acting roll and spilled all of the secrets of the film to friends, family and eventually strangers, essentially ruining the movie.
When things about new movies get leaked before the movie comes out, it dilutes the entire brand. It's twice the slap in the face when the leak comes from the movie's stars. How little respect for the director do you have to have to leak things that were given to you to make a film? If I were working on a project, and some of the actors let the material leak, they would never work for me again.
Every time a movie comes about I see people posting on social media telling everyone to not post spoilers, because they haven't been able to see it yet. I'm the same way. A lot of times before I decide to see a movie I'll simply read the summary and see who the actors are. I won't watch the trailer or any interviews that these actors have done, because they always give away hints as to what happens in the movie.
Actors, being who they are, have access to film scripts and information far earlier than the general public does. This is so they can memorize their lines, of course, and learn their character, so they can perform their role to the best of their abilities, helping ensure that the movie is successful. Everything they come to learn about the movie is their information alone, save for the film directors and other such individuals. The information these people know is a secret, and should not be leaked to the public under most circumstances. A secrecy standard, even a high one, should be met by actors. They're professional, and should therefore be able to follow professional standards. Doctors keep each of their patients' confidentiality, as do therapists and counsellors, and most big shot authors keep details of their writing a secret until publication. It is not unreasonable for actors to keep their secrets, too.