One evening a long time ago, Mark Hamill sat in an early screening of "The Empire Strikes Back." Suddenly, as a crucial plot point was revealed, from a neighboring seat Harrison Ford whipped around and exclaimed, "I didn't know that!" Ford, who played Han Solo alongside Hamill's Luke Skywalker, hadn't been apprised of the fact that Darth Vader was Luke's father until he saw it onscreen for the first time.

As Hamill recounted the story in a Reddit AMA chat on Wednesday, he revealed he was the third person on the planet to learn what was arguably the biggest movie spoiler of all time. "[Director] Irvin Kershner brought me aside and said, 'I know this, George [Lucas] knows this, and you'll be the third person who knows. If it goes out we'll know you leaked it,'" the 62-year-old typed speedily online. "I was terrified of being the one to ruin it."

Unlike one of Quentin Tarantino's script readers, Hamill stayed tight lipped. "I'm worried about Episode VII," he admitted of the upcoming J.J. Abrams-helmed continuation. "They're going after casting sheets! I'm someone who doesn't want to know what I'm getting for my birthday."

[Photos: These Earth-Bound 'Star Wars' Scapes Will Blow Your Mind]

Hamill went full disclosure, answering dozens of fan questions, revealing details on a deleted scene (that only superfans know of), and even getting buddy-buddy with Chewbacca actor Peter Mayhew, who dipped into the chat. Here are nine awesome "Star Wars" truths revealed by Hamill on Wednesday:

1. The Blue Milk of Luke's Home Planet Is Nasty.

It was ghastly! ... It's moments like that when I think, boy, I really am a good actor since I didn't gag. ... I don't know if they still make it. It's sweet and syrupy and yucky. If you want to try it go to a camping store and drop some blue food coloring in it and there you go.

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2. Luke's Pants Got Too Pricey.

I kept something from each film. From the first I kept my boots. ... I kept the Stormtrooper helmet that I rescued Princess Leia in. ... I would never think of selling these things. I saw they were auctioning my pants from the first film for $44,000 [$36,000 U.S.]! They were just bleached out Levis with the pockets ripped out. There were multiples made for me and my stunt double. That seemed like a really steep price.

3. Hamill May Post Some Good Ol' 'Star Wars' Pics, Too!

[To Peter Mayhew, who joined the chat:] I loved those pictures you released! I've got a ton of old pics from the set upstairs that I want to go through now.

[Related: Lucasfilm Gets Its 'Star Wars' Stories Straight]

4. Can Luke Protect Us All From the Anti-Christ?

The strangest [fan encounter] was a cab driver who kept telling about the anti-Christ and how Luke was blocking Darth Vader's path to taking over the world. He wanted me to help out somehow. I wasn't exactly sure how, but he was very passionate about it.

5. Pick Your Favorite Lightsaber Color.

I just worked with Samuel L Jackson and he reminded me he had a purple lightsaber, which nobody had. I thought that was cool that he got his own color. My favorite is green, I'm happy with what I had. You don't need to give me lavender or raw umber or whatever. [Luke had both green and blue lightsabers.]

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