There is a very interesting Easter Egg on Insane mode (campaign).

During Marcus' dream, when you join your team, stay close to Dom as much of the time as possible you'll know if your doing it right every time he says "Yo!").

After the battle is over and the chopper explodes, Marcus'll wake up and he'll have Dom's beard! Not just him though, all the other characters will be bearded too (even women)!

Characters will have a beard until they die.