If you've spent any time watching "Boardwalk Empire," this much you know: In a series brimming with tough characters who will ruthlessly shred your very existence without losing a moment's rest over it, no one is more disquieting than Arnold Rothstein.

This may be true of Rothstein, infamous as a gangster who knew what he wanted and did not suffer fools. But there is also this: He neither drank, nor smoked. He did, however, enjoy his cookies and milk.

Not exactly the diet of the devil. And yet it is also one of the many reasons why his portrayer, acclaimed stage actor Michael Stuhlbarg, enjoys slipping into Rothstein's skin.

"What is not captivating about him?" Stuhlbarg says by phone. "He lived so many lives in the time he had on this earth, and he was a lot of things to a lot of people, and many of them contradictory.

"You try to find as many of those grains of truth in the information that is out there. You grab it and run with it and make it your own somewhat."

Coming to the project without prior knowledge of Rothstein, Stuhlbarg dove into researching the character, reputed to have been behind the fixing of the 1919 World Series. With Vincent Piazza, who plays Rothstein protege "Lucky" Luciano on the series, Stuhlbarg patrolled New York speakeasies and restaurants in search of little nuggets to sprinkle into his performance.

"He saw a hunger in them that I think was familiar," Stuhlbarg says of Rothstein, who, in addition to Luciano also took Meyer Lansky under his wing (look for him to surface later this season). "Also, (there was) a great intelligence, a street smarts. It was very much a student-mentor type of relationship. He taught them to think of what they did as businessmen. I think it gave birth to a whole generation of criminals."

Terence Winter, the creator of "Boardwalk Empire," had been a fan of the actor's since seeing him in "The Pillow Man" on Broadway. In an e-mail, Winter describes hearing Stuhlbarg's take on Rothstein the first time and just knowing with this role at least, the casting was spot-on.

"By the time we finished shooting the pilot, I couldn't imagine having cast another actor for the role," Winter says. "Michael brings a level of commitment to his role that is truly impressive; it's almost as if he becomes another person."

Indeed, anyone who's seen the Coen brothers film "A Serious Man" and "Boardwalk Empire" would be hardpressed to say it's the same actor playing parts in both projects.

Although he once appeared in a couple of episodes of the short-lived NBC dramedy "Studio 60" a few years ago, this is Stuhlbarg's first real foray into series television. Like others in the cast, making the commitment to what could be a long-running show was made easier by the creative team behind "Boardwalk Empire," and also the chance to play a character as rich and unpredictable as Rothstein.

"Because Rothstein was sort of an enigma, it's hard to think you're going to get everything about him at any given moment," Stuhlbarg says. "I think it was a wonderful opportunity to live with a character for what could be a long time. He has such a rich history, if you're going to be committed to something for a long time, this is a wonderful character to learn about."

Contact Kevin Clapp:

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'Boardwalk Empire' airs at 9 p.m. tonight on HBO