I spotted this on gamefaqs
This Easter Egg will make every character in the game have Dom's beard, during gameplay and cut-scenes. Even NPC's will have it. To get it, you MUST be playing on Insane difficulty. Begin a new game on Insane and stick near Dom when the flashback starts, though you may need to walk away from him for a bit to get him to move at first. You'll know you're doing it right when he says "Yo" but you still need to stay near Dom for the whole flashback. It's okay to fight the Locust though, just be sure you're near Dom while you do it. When the flashback ends, if you did it right, everyone will have Dom's beard on them for the rest of the game.
I haven't had a chance to try it yet but can anyone confirm this