Reading a Screenplay

After reading the first 10 pages of Joss Whedon's Serenity, I am struck by how little dialogue there is for 10 pages of story.  I've read -- and performed and directed -- stage plays in the past and, in comparison, they are far more dense with dialogue while stage direction is often left at a minimum.  The screenplay includes a lot of what might be termed as stage directions.  It is very detailed as far as what will actually be seen on the screen, down to and including angles of some of the shots.  I had not realized how much structure there was to the screenplay, translating to a far smaller creative role than I had originally concieved of for the director.  I assume the director still has the prerogative to depart from the scripted directions in some fashion at times, however, I had thought that much of the thought for the design of the set and the angles for the cameras would have been his job, whereas now it appears that he has a far more structured plan to work off of than I thought.