Television fame remains a little surreal to Mr. Criss, given that he grew up in San Francisco, working in theater. He appeared in plays more than musicals as a teenager and college student, developing a love for commedia dell’arte in Goldoni’s “Servant of Two Masters” and notching an early romantic role as Peter van Daan, the love interest of the title character in “The Diary of Anne Frank.”

At the University of Michigan, a training ground for Broadway singers like Gavin Creel and Hunter Foster, Mr. Criss said he had never done a musical until he wrote one with friends. “A Very Potter Musical,” their parody of the Harry Potter books and movies, became a cult hit on campus; a filmed performance has drawn millions of views on YouTube, and Mr. Criss and his collaborators soon formed their own theater company, StarKid Productions, in Chicago.

In between “Glee” shoots last season, he wrote the music and lyrics for the company’s musical called “Starship,” which he referred to as “my baby.” It ran at the Hoover-Leppen Theater in Chicago last winter, drawing mixed reviews from critics but gathering a fan following on StarKid’s YouTube channel. (His moxie extends to the hope of seeing “Starship” or another StarKid show on Broadway in the next five years.)

“Starship” was typical of the “four or five projects” that Mr. Criss said he had going at one time, relying on e-mail and iPhone applications to help him stay working beyond “Glee” land. In fact, he described his eight-performances-a-week schedule in “How to Succeed” as “the easiest thing I’ve done in a while, in the best sense,” compared with “Glee” shoots that can begin at 6 a.m.

“This lifestyle isn’t so different from when my mom drove me from theater rehearsal to soccer practice to violin lessons in a single day,” said Mr. Criss, whose boyish face was masked by modish black-rimmed glasses and the light beard he often grows when he’s away from “Glee.”

Even though he looked different from his “Glee” character, Blaine Anderson, a succession of fans came over in the diner to ask for autographs, including three tween girls from Buenos Aires who were briefly dumbstruck when he asked how to spell their names.

Mr. Criss credited his parents with “giving me a sense of humility and the good judgment about how to spend every day.” Those instincts, he said, led him in 2010 to meet with the film and theater producers Craig Zadan and Neil Meron, who were impressed with his “Glee” work.