"Obi-Wan never told you what happened to your father..."
"HE TOLD ME ENOUGH!... He told me YOU killed him..."
"No. I am your father!"
"No. That's not true. THAT'S IMPOSSIBLE!!"
"Search your feelings, you know it to be true!"
What you DIDN'T know is that this page of the script was actually a joke page that someone slipped into the script. Vader wasn't originally supposed to be Luke's father, but it was so good that they kept it.