Facial muscles are generally located quite near the surface. This means that their activity may be measured by means of surface EMG (electromylograms) in which the electrical activity is measured by means of electrodes on the skin instead of inserted needle probes.
A facial waldo  is a device for direct electro-mechanical sensing of facial movements. The advantage of this technique is that very little processing of the raw data is required. The disadvantages are low spatial resolution resulting in crude facial movements, extensive calibration procedures, and intrusiveness.