Using the right shutter speed, when shooting movies, is important to the look of your movie. A slower shutter speed will blur motion similar to what happens in photos and a fast shutter speed can result in really sharp frames but can make motion look a bit jittery. Technically I should be referring to shutter angle seeing as how we are talking about movies and this relates to film and how it is exposed. The recommended shutter angle is 180 degrees which basically means that if you are shooting 24 frames per second then you shoot at 1/48 of a second, 25 fps - 1/50 of a second, 30 fps - 1/60 of a second, 60 fps - 1/120 of a second etc.
I found this article at inventerspot.com it explains shutter angles much better than I and also has other useful links.
The trick is how to shoot with a DSLR and keep your shutter speed constant! I have found that a Variable ND Filter is the best way to do this. I have a ND2 to ND400 Filter, you rotate it like a Polarising Filter which means once you have your Aperture and Shutter Speed set you rotate your Variable ND Filter until you achieve the exposure you want, simple!
Variable ND Filters are not expensive and are an indispensable piece of kit for the DSLR movies shooter!