Here goes, I use Plastic Man a lot on my DC team and I have mistakenly underused his shape change power because I was under the assumption it was a ####ier version of super senses.
But after looking at the definition its got my mind stirring up some questions.
When this character is chosen as the target of an attack, roll 1 six-sided die. On a result of 6, the attack cannot be made; the character has fooled the attacker by appearing as a friendly character. The attacker must choose another target or use that character to take a different action instead.
Does this mean that you can roll for this before or after the attack is made on Plastic man? Just as long as the player says X to attack Plastic Man. And does this mean I can roll for SC even if the attack is not succesful?

The roll for Shapechange should be made after the declaration, but before the dice are rolled.
So, if someone says they will attack Plasticman... it is at this point the roll would be made. If your roll is successful, then he's got to choose another target. If your roll fails, then he can continue with the attack.

The Shapechange power basically works exactly like the Skrull team ability.