Keanu Reeves — yes, Keanu Reeves — hosts a fascinating documentary about the impact of the switch from film to digital images in movies, tonight on PBS, and anyone who loves movies should take a look. For me, the revelation had nothing to do with the quality of the image, but the good and bad consequences of allowing a digital camera to run for hours without stopping, while film cartridges had to be changed several times an hour. Some actors, in some circumstances, love the chance to stay in the moment and keep trying new variations on the performance. Some directors keep the camera going and wear the actors out. As film begins to disappear in movies the way it has in still cameras, this thoughtful and insightful documentary illuminates what that means.