In the past they've posted in the Daily Planner what movies will be playing, and what time. They do tend to play lots of the same stuff over, and over, but the better movies are shown in the evening. When MUTS first started, you had to get a ticket for the evening showing, but the last time we were aboard, they had stopped requiring that. MUTS is great, and I think you'll enjoy it a lot.
Have a great time aboard.
I just got off the boat! It was one of the best cruises I have ever had and my 3rd on Princess. To answer your question, my impression was that they had a feature movie every night, never repeating anything. During the day however, I did notice some movies or other shows repeated. I would have loved to have done MUTS, but there was just so much to do, that I didn't have a chance. Maybe next time for me. It was such a good cruise!