HOLLYWOOD - The job seems like it should have been a cakewalk for three of America's most gifted comedians: a couple of weeks in a recording booth voicing laughable yet lovable characters for a big-budget, computer-generated animation adaptation of the beloved children's book “Horton Hears a Who!”

So it was reasonable for Jim Carrey, Carol Burnett and Steve Carell to expect the project to be one of those windfall gigs that accompany a certain strata of Hollywood stardom: fat paychecks and critical props from kids everywhere for what is essentially a performance they could give wearing sweatpants and no makeup.

But to listen to them recount the experience, the actors' vocal contributions to the film version of “Horton!” resulted in feelings of frustration, isolation, creative confusion - even bodily pain.

“It was much more complicated than they promised it was going to be,” Carrey said, sucking down coffee at Beverly Hills' Four Seasons Hotel on a recent Sunday. Sitting across from him, Burnett and Carell nodded in agreement. “At the beginning, they come and say, ‘A half an hour and you're out of there,' ” Carrey continued. “Months later, you're taking another Saturday. It was a lot of work. I was sorer leaving the studio some days - throat raunched and physically drained - than if I was in front of a camera.”

Not exactly what you expect to hear from a guy who, early in his career, made millions pretending to talk out of his rear end - and for whom goofy, free-form volubility is a default setting. But Burnett, a comic icon and enduring TV star (anointed an American Master on a PBS special last year), and Carell, who has flexed his funny on NBC's “The Office” and in films such as “The 40-Year-Old Virgin,” echo his assertion that making “Horton” was no laughing matter.

“It's a little difficult to admit that it was hard,” Carell said. “To me, the most difficult aspect was trying to find what the directors wanted. I didn't have any interaction with any of the other characters. I didn't know how Jim would be saying a line. I didn't hear any of the playback. What was so strenuous was trying to get as many completely different reads as possible. Because there's no frame of reference.”

Burnett accepted the role in large part because of Carrey and Carell's participation. For her, acting in almost total isolation was anything but Who-larious.

“I got very frustrated at one point,” she exclaimed, accidentally knocking over Carrey's drink. “See? I told you I was mad.”

In the film as in the book (first published in 1954), a giant, overzealous elephant named Horton (Carrey) overhears a tiny scream coming from a tiny white speck being carried by the breeze. Fearing it might house some infinitesimally small family in jeopardy, he vows to keep the speck safe from harm despite the jeers of friends and the scorn of a vindictive jungle elder, Kangaroo (Burnett), who pressures Horton to ditch the speck.

Turns out it's home to a microscopic town called Who-ville, whose bumbling mayor (Carell) entreats Horton to carry the Whos on an epic journey to safety - even if that means Horton must incur Kangaroo's wrath, not to mention the murderous vulture (voiced by “Arrested Development” alum Will Arnett) she hires to track him down and destroy the speck.

Adapted for the screen by co-directors Jimmy Hayward and Steve Martino (who lent their computer animation experience to the 2005 sci-fi cartoon romance “Robots”), “Horton” is the first CG-animated film to be based on any work by Dr. Seuss - ne Theodore Seuss Geisel.

“I'm just so honored to be part of his legacy at all,” Carrey said of Seuss. “I'm just so lucky.” He paused: “I just hope he doesn't punch me out when I go into the next realm.”