Whoa. George O'Malley, is that really you?!

"Genius" and they told him it was for the part of J. Edgar Hoover. But then they explained that he ages 40 years across the series, requiring a huge physical transformation.

National Geographic's "Genius" centers around the life of Albert Einstein (Geoffrey Rush), the titular genius who was loathed by FBI director J. Edgar Hoover. Knight talked to Access about playing Hoover, a man he considers a "monster" in real life, although he tried not to focus on that when playing the character. Knight said his transformation into Hoover took five hours in the makeup chair, and you can watch a sped-up version of the process in the video below.

And for the record, yes, Knight still gets recognized all the time for playing George O'Malley on "Grey's Anatomy" -- and he's cool with it. "It's so great when you have people -- sometimes when the younger people, it's surprising because there's so much television out there, that they go back and watch it. And I always want to know if it's dated to them. .. But it's exciting. I think it's a testament to Shonda's [Rhimes] work."

Knight said "Grey's" was his first experience with prosthetics, and they used the same techniques in "Genius."

Watch the whole interview:"Genius" continues Tuesday, June 13 at 9 p.m. on NatGeo.

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