Facial twitch: I believe these twitches are similar to the twitches people get in their eyelids. Usually it goes away within 6 weeks or so. There are other causes such as caffeine, stress, alcohol, electrolyte imbalances and excessive fatigue. Feel better.
A shutter speed of 1/30 for example will let in twice as much light as a speed of 1/60 since it is two times slower. When you decrease the shutter speed by a factor of two you are increasing the amount of light by one stop. Remember that fractions get larger as the number on the bottom gets smaller.
I've always played with the rule of keeping close to double the frame rate, or a multiple higher (i.e. shooting at 30 fps, 1/120 shutter speed is fine). @multimediamarketinggroup: that has to do with the frequency of the artificial light you're shooting in, which is 60 Hz in the US and 50 Hz in most of the world.