On the April 5 episode of CBS’s The Big Bang Theory, Sheldon (Jim Parsons) gets a visit from renowned theoretical physicist Stephen Hawking.

I spoke with Jim exclusively about the episode at the Alzheimer’s Association’s Night at Sardi’s event and learned that the actor, like his character, was blown away by meeting one of the world’s most intelligent people. Special steps had to be taken to accommodate Hawking, who suffers from a motor neuron disease related to amyotrophic lateral sclerosis, a condition that has left him almost completely paralyzed.

“It was insane and intense, if I’m being honest,” Jim says of the shoot. “To be in his presence was a very powerful thing. He’s communicating only through a speech-generating device, but I was also able to look into his eyes.”

“We had this scene written out, and obviously his lines were pre-programmed into his computer,” Jim continues. “They had the capability of just cueing his lines, but he demanded to control every take himself by operating his machine with his cheek muscle. At first he was very slow getting the cues going, but they fine-tuned it. Only at the very end of the day did I realize the magnitude of what we’d pulled off.”

In the episode, Professor Hawking comes to lecture at the university where the guys work. “Wolowitz is on the engineering team to make sure everything stays okay with his wheelchair while he’s there,” Jim says. “Sheldon asks Wolowitz to give Stephen Hawking a paper Sheldon has written on the Higgs boson particle. It’s a long and winding road to get to the point where I’m actually allowed to see him, but it happens.”

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