Currently Matt Damon is filming a movie in my area. A friend’s business is being leased for a few scenes. It’s really pretty cool.
My friend has taken some pictures. She posted one on her FB page. Within an hour she was contacted by movie people telling her to take the pic down until the filming is finished. They are holding her final check for reasons like this, so of course she complied.
NDAs or “Non-Disclosure Agreements” are plastered all over the filmmaking process. IANA filmmaker but as I understand it, it’s partly because film companies and actors (and their agents) like to be the ones to decide whether and which information/images about them and their activities will be made public. (Of course, they can’t completely control the activities of paparazzi and so forth who are under no obligation to them, but they can certainly lean on people like your friend who have a contractual agreement.)
And it’s also because publicity and marketing for films are focused on very controlled release of information. You never know until after the editing process is finished exactly what shots are going to be in the movie and exactly what their importance will be. You don’t want it to turn out that one of the significant “reveal” shots in the movie relies on some detail on the set that is supposed to be a surprise but that all the audiences know about because it was part of some blogger’s photo posted online during shooting that went viral for some reason.
The production of Return Of The Jedi was done under the codename/working title of Blue Harvest, because Lucas and others felt that they were ripped off during the location shoots for Empire, the Star Wars name had become too well known and ROTJ required further location shoots.