Spoilers for Westworld Season One to follow.

One of Westworld's most popular fan theories has finally been confirmed. William—the white hat played by Jimmi Simpson—is the younger version of the sinister Man in Black played by Ed Harris.

The William/Man in Black twist had been suspected by many of Westworld's eagle-eyed fans, but one of the few people who knew for sure was Jimmi Simpson. Speaking less than an hour after the Westworld finale ended, he was clearly adjusting to being able to talk about his role in the HBO drama so openly. "It is strange," he told me. "But it's nice to be able to talk about the whole story."

What's it like being at the center of the biggest twist in HBO's buzziest new drama? Jimmi Simpson talked to GQ.com about fan theories, the Man in Black, and the role William could play in Westworld's future.

How much did you know about the extent of William's transformation when you signed on to star in Westworld?

He wasn't really described at all. [My audition] was a couple of scenes of a really sweet guy—and then one scene where he seemed to have had enough of being pushed around. So all I knew was that this was a really nice guy who has been brought into this extreme circumstance because of another man's means, and money, and motivation. He's kind of there unwillingly.

When did you find out you were essentially playing half of a much larger character arc, with Ed Harris handling the other half as the Man in Black?

I found out around…I guess episode three or four, when our amazing makeup designer, Christian, was asking about changing the shape of my eyebrows. And he wouldn't tell me why. I could only deduce that the only reason you would change a person's eyebrows was to try to make them look more like someone else. At least for a man, you know? No one has ever asked to change the shape of my eyebrows.

So I was walking along, a couple hours later, to lunch with [showrunner Lisa Joy]. I didn't think too much of it, but when I did think about it, the only possible person I could turn into—as far as I was concerned, given dialect and eye color—was Mr. Harris. And I brought it up to Lisa. I said, "Hey. Christian was just asking me about my eyebrow shape." And she got kind of stiff for a second. And I go, "I mean…am I gonna turn into Ed Harris?" And she stopped talking and she goes, "I can’t answer that, and don’t ask me that again." [laughs] Which confirmed it for me. But then she was clear. She said, "Look. Your arc is huge. Just please don't share it with anybody." And I didn't. I didn't share it with a soul.

Other than the eyebrows, was there any effort to keep your William and Ed Harris's William consistent?

I took it upon myself to ask [Jonathan Nolan and Lisa Joy] for any clips [of Ed Harris] they'd be willing to share. And on the couple of days when Mr. Harris and I worked on the same set, I just kind of…stared. And watched. I tried to be as subtle as possible, and I was leaning into the narrative more than my specific physical appearance. But as William’s will is broken and his morality is tested, the weight is visible on him. I felt like you could see that on Mr. Harris, so I tried to drop that into [my performance in] the later episodes.