The situation where Evil Twin is reversed -- the Evil Twin isn't pretending to be the character, but instead the character is pretending to be the Evil Twin. Never mind that the original character hasn't studied the Evil Twin's typical mannerisms, knows nothing of his background and may not have his abilities. Never mind that the Evil Twin may be wearing a mask or equivalent which the real character can't take off since it's his real face. They look alike, so it has to work, right? In fact, it works more often than it should, often stopped only by gross problems (such as the real Evil Twin walking through the door), not by the dozens of flaws in the impersonation that nobody notices. Frequently it works because the Evil Twin's Mooks are too scared of him to question his changes in behavior