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.
ISO 400 film needs a quarter of the light that ISO 100 needs. In other words, you could capture a low-light scene with a shutter speed of 1/15 second with ISO 100 film, or 1/60 second with ISO 400. That's an incredibly powerful capability that means the difference between getting a blurry mess and a sharp photo.