When Vader attacks a Character other than a Jedi Knight or Jedi Master, you may pay Force equal to that Character's total build cost. If you do, that Character retreats when Vader's attack ends.
Pay 2 Force -> Evade 2 (Prevent up to 2 damage to Vader.)
Pay 1 Force -> Intercept (If a unit is attacking one of your Characters in the same arena as Vader, it now attacks Vader instead.)