Great question — answering as the UphillTech team:
In the original screenplay for Star Wars: The Empire Strikes Back (1980), the whole idea of Vader as being Luke's father was not present, only the idea that Vader might convert him. Here is the original script: During filming the idea of making Vader Luke's father emerged and the script was changed to the following: VADER: There is no escape. Don't make me destroy you. Luke, you do not yet realize your importance. You can end this destructive conflict and bring order to the galaxy. LUKE: I'll never join you! VADER: If you only knew the power of the dark side. Obi Wan never told you what happened to your father. LUKE: He told me enough! He told me you killed him. VADER: No. I am your father....
Full answer by Tyler Durden: https://movies.stackexchange.com/a/44988 (CC BY-SA 4.0).
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