Crepe hair in a few similar colors (the one you want, and one slightly darker and lighter). Spirit gum to attach it.
Grab a fine toothed comb, and pull a couple times through the main color, once through the darker, once through the lighter... repeat.
Grab the protruding hair between the sides of the fore and middle finger of one hand, and cut with a scissors between the comb and fingers...
Spirit gum the actor's face (if a larger application, make sure to start at the middle and bottom and work your way up and out). Once the spirit gum is tacky, apply the crepe hair in your fingers, cut side to the face and hair dry gently (burning actor's skins bad) to fix the adhesive... repeat ad (grizzly adams) naseum.
Once the unkempt mountain man beard has been built, trim it like any other beard - Should make a beard that looks much more realistic and fits and contours to the actor's face...
To get it to lay down better, as you touch the cut ends to the face, move your hand slightly down to get the individual hairs to lay down rather than sticking out.
This takes time... it looks great though. We did this after an actor had shaved in the middle of a shoot to fix that particular problem... I still can't tell the difference in the screen shots other than knowing when he had done each of the shoots.
