I am on the Mariner right now and the movie theatre is right down the hall from me. It is called "The Screening Room" but us actually quite small. They offer movies during the day and evening. I have not gone to one, but it is there and quite intimate.
They also have channels with movies and pay for view available in the cabins.
Hope that helps. Oh yes, The Screening Room is on deck 2 all the way forward and has a large screen with the movies being shown for the day and the times at the very front of it.