As this is happening, we see a parallel story of Anakin sliding to the dark side. He's broken and angry, and the Emperor plays on his vulnerability much as he actually does.
The Emperor rescues Anakin, puts him in the Vader suit. He's already got the mask because he had to hide his identity! He becomes the symbol of fear for the Empire, the Emperor's strong right hand.
So when Episode IV rolls around and Obi-Wan tells Luke "my pupil Darth Vader killed your dad," the audience is like WHOOOOA, WAIT A SECOND THERE, BENNY.
Now we have misdirection.