Playing HD movies can put quite a strain on your CPU. If the system is a bit older you can experience stuttering playback. If that's what you're referring to then apart from the obvious solution, a faster computer, there are a few things you can do.
'Things' that run in the background take up CPU time. Closing other running applications will free up more CPU power for the application playing video.
Keep your codecs current. A newer codec might be more efficient than an older one.
Use something like Process Tamer
or similar utilities to give your video player a higher priority. 'Above Normal' could be enough to get smooth playing video. You can set so called 'explicit rules' in the right column of this program that will force an application's priority every time it is run. Quite convenient as you can most likely keep other applications active and don't need to shut them down.
Let us know whether this solved your problems.
When in doubt..... Press the shutter.