AFTER seeing the “The Normal Heart” on Broadway last June, three teenagers from Minnesota were in a frenzy explaining why they had chosen the play. “Sheldon!” they all shouted, naming the socially clueless lead character on the CBS hit comedy “The Big Bang Theory” played by Jim Parsons, who had a small role in the play.

Here was celebrity casting in action, yet it had unintended consequences: the teenagers hadn’t known that the show was about gay men dying of AIDS, and they left disappointed that Mr. Parsons wasn’t acting as outrageously pompous as Sheldon, a role that earned him Emmy Awards in 2010 and 2011.

Mr. Parsons is back on Broadway during another summer hiatus from television, and this time he faces audience expectations that are even more complicated.

Not only is he dealing with the shadow of Sheldon again, but also that of a certain actor named Jimmy Stewart. Mr. Parsons is leading a Broadway cast for the first time, in a revival of “Harvey,” the Pulitzer Prize-winning comedy from 1944 about the sweet-natured Elwood P. Dowd and his invisible friend, the title character, a 6-foot-tall rabbit. The show ran on Broadway for four years, opening with Frank Fay as the lead. Stewart later followed as Elwood before bringing him to wider fame in the 1950 film, for which he was nominated for an Oscar. He reprised the role in 1970, the only previous revival on Broadway of this play, which became feared for the task of taking on The Jimmy Stewart Role.

While Mr. Parsons is keenly aware that both Sheldon and Stewart are indelible, he has also drawn confidence from acting techniques and instincts that have served him well for more than 20 years, since choosing his career after performing in the farce “Noises Off” in high school.

“People may not like me as Elwood, people may say ‘I enjoyed Jimmy Stewart more,’ ” he said recently, over coffee at a Midtown Mahattan hotel. “There’s nothing I can do about that. But I have to come in and take a stand on the performance, as it were.”

He memorizes his dialogue well beforehand, writing out lines on white 3-by-5 index cards (he has 200 for “Harvey”). He creates precise physical worlds for his characters, down to where they would place a hat or coat or, in the case of Harvey, where the rabbit would be at every second. He obsesses over body language too: the angular, ungainly stride he created for Sheldon, and the alternately swift and halting paces of Elwood.

And he still has never seen the Stewart film or any stage production of “Harvey.”

“I try to master every facet of a character in order to build a safety net for myself, so I can go on to take more risks to create someone really distinct,” said Mr. Parsons, who is 39, roughly the same age as Stewart when he first played Elwood on Broadway.“One of my very early teachers said over and over again: ‘What are you bringing to the party?’ That expression never left me.

“Well, what about it? What am I bringing to this party?”

“If I’m not making a choice with each and every line,” he continued, “then why are you bothering watching me?”

Mr. Parsons hastened to add that he was not disrespecting Stewart, nor was he cavalier about audience reactions. He is the sort of person, in fact, you could imagine taking his bow then apologizing to theatergoers if any of them were disappointed with his work. His unfailing politeness has an old-fashioned courtliness to it; at a rehearsal for “Harvey” this month, he said sorry every time he had to ask the script prompter to remind him of a line.

It sounded almost automatic, reflecting a tendency to speak his mind without a trace of self-consciousness (a habit that makes his television character so winningly exasperating). At one point during the rehearsal, for instance, the actress Jessica Hecht — who plays Elwood’s sister, Veta — put a prop down on stage in a spot where Mr. Parsons wasn’t expecting it.

“Did Jessica leave that there?” Mr. Parsons said. “That’s never going to work.”

“It won’t happen again,” Ms. Hecht replied collegially.

“It never happened before,” he said — not in an accusatory way but, like Sheldon, with an almost absent-minded bluntness.

“Um, let’s talk about it more,” she joked.

Later, Ms. Hecht described the way Mr. Parsons speaks as a kind of afterthought.

“His concentration is so total that he sometimes says surprising things that I don’t even think he’s aware he’s saying,” said Ms. Hecht, a Tony nominee for her last Broadway role, in the 2010 revival of “A View from the Bridge.” “There’s something so dorky in the best way about him.”