Warning: Spoilers ahead for season one of Westworld.
From the minute the Man in Black (Ed Harris) was introduced in the premiere of HBO’s Westworld, his identity became the primary subject of online conversation about the show. Who is this man? What does he want? What’s his background?
When viewers were then introduced to a new arrival named William (Jimmi Simpson) in episode two, fans started theorizing almost instantly that the two were connected.
Over the course of the season, William was told several times that the park would reveal his true self — perhaps indicating his journey from a "white hat" to a "black hat"— and there was ample evidence from the very beginning that William’s story took place in a different era than the Man in Black’s. Could they be the same person? Of course, Sunday night’s episode revealed the connection between William and the Man in Black, but it was just the end of a journey.
What were the clues that got us to this point? Follow us down the rabbit hole. (Shout out to Joanna Robinson’s excellent work at Vanity Fair for some of the clues and links below.)
"Do you remember?"

From his very first scene, William was subconsciously placed in a different era of the park by the show’s creators. The second episode opens with Bernard (Jeffrey Wright) telling Dolores (Evan Rachel Wood) to wake up again, and then we hear “Do you remember?”
in voice-over as we see a sleeping William on the train on his way into Westworld. They may as well have put a big flashing sign that said “flashback” on the screen. Looking back on it now, there are several clues in that episode that William’s journey is starting in a different era of the park.
The station he arrives at looks a lot like what we saw in the waterlogged, cold storage facility from episode one. The Westworld logo is different. And when William and Logan (Ben Barnes) take the train into the city, they don’t encounter Teddy (James Marsden) on his loop as they likely would have if he were in the same narrative as the premiere. So we knew early on that William was in a different era of the park than the Man in Black, but that didn’t mean he was the Man in Black. How else could we have known?
It's all about Angela.

Who would have guessed that the host who greets William on his arrival would be our best indicator of the shifting time frames of Westworld? (Seeing Lawrence first in the Man in Black’s timeline and later in El Lazo in William’s was further evidence of different timelines, but Angela was the real anchor.)
We saw Angela in three forms. In flashbacks, she’s dancing in the town square in the very early days of the park, before it’s even open to the public, when Arnold and Ford were tweaking the hosts.
She greets William on his arrival in the second episode as Angela. Finally, she pops up as the traveler who the Man in Black meets and clearly recognizes in episode eight. “It’s you, I figured they retired you,” he says to her, indicating that he remembers her but hasn’t seen her in years. For many, this interaction was the smoking gun for the theory that William was the Man in Black. He recognized a host he hadn’t seen in years, and it just so happens to be the same host who first greeted William? Case closed.
William's fiancée and the Man in Black's wife

In episode eight, the Man in Black reveals his backstory, admitting that he married a woman 30 years ago, and that her recent death sent him back to Westworld.
In William’s timeline, we knew that he was about to marry Logan’s sister, Juliet, and multiple clues hinted that William’s arc was taking place 30 years ago. We get little exposition about either character’s life outside of Westworld, and yet the two things we learn are that William was about to get married, and that the Man in Black’s wife recently died.
They were clearly designed to be book ends, cluing us into the fact that this was the same character. The Man in Black also spoke of wealth and great success on multiple occasions, while William was about to marry into money. By this point, if William weren’t the Man in Black, it would have felt like a cheat.
See the rest of the story at Business Insider