As an actual stills photographer, I would say knowing and being good at stills is a good starting point for a cinematographer but cinematography in general is much MORE than still photography: motion (camera, light, subject and background) and the capture of it is more complex than simply a series of stills. You have to be cognizant of the interaction of all 4 in your scene and how they can be best arranged for maximal effect for the story you are trying to tell.