In an interview with StarWars.com, George Lucas explained how the “Star Wars” cast and crew tried to keep the truth behind Darth Vader a mystery. The filmmaker ensured the plot didn’t make it to the scripts, and he didn’t share the information with anyone. Mark Hamill was only informed of his onscreen father’s presence on the day of filming — there was no room for anyone else to find out!

“… The thing about it is I didn’t tell anybody — anybody — about it. And it wasn’t in any of the scripts. It wasn’t even in the story treatments. I kept that aspect of it secret and I was the only one that knew about it. And it really wasn’t until the day we shot that we told Mark [Hamill] so he could react appropriately.”

The film’s director, Irvin Kershner, was the one to inform Mark Hamill that only three people knew about Darth Vader’s secret, and if the world found out, he’d know who spilled the beans.

He said: “‘I know it. George knows it. And when I tell you, you’ll know it,'” Hamill recalled. “‘But that means, that’s only three people. So if it leaks, we’ll know it’s you.'”