T.R. Knight marches on with ‘Parade’

The musical, which opens tomorrow at the Mark Taper Forum, is based on the true story of Leo Frank, lynched for the murder of a young girl who worked in the factory he managed. Knight plays Frank.

Knight said he’s grateful for everything he learned on “Grey’s Anatomy.’’

He said he watched this season’s premiere of the ABC medical drama, which bid goodbye to his character, Dr. George O’Malley.

“It was fascinating . . . not knowing what was going to happen and being on that side of things,’’ Knight said. “They do just beautiful, beautiful work. If that doesn’t sound condescending, I was so proud of them. It was a beautiful episode.’’

Although Knight appeared in “Noises Off’’ on Broadway (2001) and in the 2003 off-Broadway production of “Scattergood,’’ he’s never done a musical as a professional actor.

“I did it when I was a teenager, but . . .’’ he breaks out in song, “I’m not a teen-age-r an-y-more!’’

So Knight, 36, had to learn to act while singing.

“When I moved from singing with my voice teacher and then with the music director, and then you had to do it in front of the cast, each of these kind of steps has been, OK. Now I’ve got to get through this, got to get through this. One day, when I was singing and I had this larger note and it just went, Ahhh!!! I don’t know what it sounded like. Goat-ish, kind of? Those are moments where you go, I did that now. I totally cracked in front of the entire cast and, OK, at least that’s done.’’

“Parade’’ director Rob Ashford says he wasn’t looking for a big voice.

“It’s about the inside of this man that bubbles out,’’ Ashford said. “I knew [Knight] was that kind of actor.’’

“Parade’’ costar Charlotte d’Amboise says Knight is “incredibly smart and sweet and generous.’’

Knight said he isn’t sure what his next move will be after “Parade’’ passes by.

“Regardless of the medium, I just want to work with people that - I mean, if I’m lucky enough to work. Every actor has that: This will be the last time. This will be it. If I can be greedy, I’d love to work just with people I admire and respect. That would be the best.’’

The Center Theatre Group production, which comes from London’s Donmar Warehouse, runs through Nov. 15.