THE alarm clock woke Michael Cerveris with a Kurt Weill recording at 7:30 a.m. on a recent, blustery Tuesday morning. By 9 Mr. Cerveris, the “Sweeney Todd” star, had showered, dressed, checked e-mail and was out walking his dog, Gibson. His distinctive shaved head covered by a stocking cap, he ducked into a deli and bought a healthful-looking green juice, which he consumed in his living room with a bowl of organic oatmeal and some vitamins. Crouching down, he gently fed Gibson a few vitamins of her own, dipping them in peanut butter.

Michael Cerveris and Gibson after visiting a deli to buy juice for breakfast before a rehearsal of “LoveMusik.”

Mr. Cerveris was determined that both members of his household stay healthy over the grueling course of the next few weeks, when he would split his waking hours between two major New York productions. By day he would rehearse a new leading role (as Weill in Harold Prince’s forthcoming Broadway musical “LoveMusik,” with a book by Alfred Uhry). By night — and matinee — he would perform Shakespeare (the Earl of Kent in James Lapine’s “King Lear,” at the Public).

Between seven “Lear” performances and six “LoveMusik” rehearsals a week, his only time off would be a few hours on Sunday mornings and Monday evenings. The other five days of the week, his workday would begin before 10 in the morning and finish after midnight.

Sitting in his cozy, funky living room against a wall lined with vinyl records, discs, books and vintage guitars, Mr. Cerveris assessed the situation. “In general, scheduling’s always a little fuzzy in this business, so it’s not surprising that two projects I was doing ended up overlapping,” he said. “Then you add to that my particular refusal to miss anything. And then,” he added with a wry smile, “I seem to have this belief that I’m superhuman.”

Even under ordinary circumstances Mr. Cerveris’s stamina is the stuff of legend. (He didn’t miss a single performance in a year of “Sweeney Todd.”) But he is also an old hand at pulling theatrical double duty.

There was that stretch in 1992 in California when he was rehearsing “The Who’s Tommy” until 3 p.m. in La Jolla, then driving up the clogged freeways to make his “Richard II” curtain at the Mark Taper Forum in downtown Los Angeles. Over the course of his 1,403 performances in “Tommy,” Mr. Cerveris — who is also a serious rock musician — played the occasional two-hour live set after the show. For four weeks in 1998, while appearing in “Titanic” on Broadway, he was rehearsing to take over the title role in “Hedwig and the Angry Inch.” And then there was that brief 2004 stint when he played a French libertine Off Broadway while rehearsing his Tony-winning turn as John Wilkes Booth in Stephen Sondheim’s “Assassins.”

Most recently, during a February performance of “Lear,” Mr. Cerveris took advantage of his character’s 45-minute offstage break to pop down to Joe’s Pub, where he sang a Lee Hazelwood song — in his costume — before heading back upstairs for the last act. (“That was a little nerve-racking,” he admitted.)

The son of a Juilliard-trained music professor and a modern dancer who settled in West Virginia, Mr. Cerveris grew up immersed in various art forms. “Ever since I was a little kid, I’ve felt like I’d rather sacrifice sleep or meals than miss out on something good,” Mr. Cerveris said. “I always figure I can sleep later.”

So on this wintry Tuesday, Mr. Cerveris set off briskly for the subway. He arrived at Manhattan Theater Club’s rehearsal studios a few minutes before 10, hung up his parka, put on his glasses and opened his three-ring binder on top of the upright piano. His solemn, concentrated air suggested his “LoveMusik” character, the German-born composer of “The Threepenny Opera.” Mr. Cerveris sang a Weill song in a light, German-accented voice. By the time he joined his co-star, Donna Murphy (who plays Weill’s great love, the fabled Viennese singer-actress Lotte Lenya), for the intimate “Speak Low,” rehearsal had begun in earnest.

After four hours and dozens of songs Mr. Cerveris bundled up against the cold and went out to buy lunch at an organic deli. It was 2 p.m., and he still had nine hours of work ahead of him, but he was in good spirits. “The tricky thing,” he said, “is going to be making sure that Kent doesn’t wind up with a German accent.” All kidding aside, he said, some similarities between Kent and Weill — both loyal men, with a keen sense of right and wrong — were beginning to work on his imagination.

Back at the studio Mr. Cerveris was tapping out e-mail messages on a Treo when a very animated Mr. Prince rushed in and threw an arm around him. “I was thinking about the bathrobe,” Mr. Prince began, inspired by a new costume idea. After he had left, Mr. Cerveris shook his head. “I still can’t believe Hal Prince even knows who I am,” he said. “To work with him, I would’ve played a tree in ‘The Johnny Appleseed Story.’ ”

Mr. Cerveris’s last two Broadway appearances have produced Tony nominations, and no less an authority than Stephen Sondheim says he “can do anything.” Oskar Eustis, the Public’s artistic director, calls Mr. Cerveris “both a true star of the musical theater and an actor able to inhabit the classics with a spirit as big as anyone on the New York stage.” On the other hand, as Mr. Cerveris pointed out during his evening commute, it is tough to develop a swelled head when you are hustling on the N train from rehearsal to your 7 o’clock curtain.

Juggling a banana and a cup of herbal tea, he reviewed a copy of “King Lear” as the subway rolled toward Eighth Street, reacquainting himself with Kent. Emerging on the sidewalk at Astor Place, he exhaled deeply. “I’ve always felt the most at home downtown,” he said.

As he came through the doors of the Public at 6:05, his tightly scheduled day took an abrupt, unexpected turn. The evening’s “Lear” had been canceled, due to a last-minute cast-member illness.

Instead of performing “Lear” he found himself in a three-hour emergency rehearsal with the production’s understudies. Arriving home after 10, he walked his dog, answered e-mail and spent an hour putting the pages of his binder in order for the next morning’s musical read-through. Then he remembered that he had offered to play the Talking Heads song “Psycho Killer” at a forthcoming benefit. So he watched a bit of the concert documentary “Stop Making Sense,” to see how difficult the guitar part was. “It looked doable,” Mr. Cerveris said.

By 10 on Wednesday morning he was “essentially performing ‘LoveMusik’ sitting down” at the first full-cast read-through, which ended at 12:23. By 12:45 he had walked the dozen blocks to a callback for a television gig — trying, he said, “to squeeze in half an hour for career maintenance.” At 1:03 as he was about to give up, he was whisked into his two-minute audition. He jumped into a cab and made his Wednesday matinee check-in with a few minutes to spare.

It was a routine matinee. During its three-plus hours Mr. Cerveris wrestled with a knave, was put in the stocks and drenched by a rainstorm. (At the end of the third act he retired backstage and waited in a bathrobe while his costume went through a clothes dryer.) Offstage he worked a crossword puzzle; only during his 45-minute break, when he had “enough time to go away mentally and come back,” did he peek at his Weill research.

Sitting in the Public’s lobby during his 90-minute break between the two Wednesday shows, Mr. Cerveris reflected on the two characters now sharing space in his head. “You know, to me, the demands of the roles are not that different,” he said. “I’ve always approached songs as an actor — as if they were monologues on pitches. So I don’t find it that strange to go from ‘LoveMusik’ to Shakespeare.”

He was used to squeezing his social life into the afternoons, but while doing double duty it had to be crammed into even smaller margins. “But I think of acting as a service profession,” he said. “There are these moments when I’m part of this group, creating this unique, unrepeatable experience and putting something valuable into the world. And it’s at those moments that all the sacrifices I’ve made are justified — if they can be justified at all.”

Eight days later, over his “LoveMusik” lunch break, Mr. Cerveris exuded energy. His “Lear” performances, he observed, had been feeling increasingly free, “as if, after spending all day in creative exploration, I’m able to walk onstage and meet the play where I am at that very moment.” Likewise, in “LoveMusik” rehearsals, he had been making bolder-than-usual acting choices. “I’m editing and monitoring myself less while I’m doing it,” he said. “And I think that’s a good thing.”

Over the past week too his understanding of the qualities shared by his two characters had deepened. “They’re both extremely devoted,” he said. “They sacrificed and asked very little in return. The duty itself: that’s the reward.”

He paused, considering. “What I get out of acting is kind of like that,” he said quietly. “I mean, sure, there are rewards for what I do. But in the end if the devotion itself doesn’t mean enough to you, I don’t think there are enough rewards in the world.”