Mark Walton, an animator for Disney, is the unlikely voice of Rhino, an overweight, television-obsessed hamster shaping up to be the breakout character in Bolt, in theaters now.

WALT DISNEY PICTURES

As a visual development artist for Walt Disney Animation Studios, Mark Walton normally toils far from public view. His cubicle, located in a remote corner of the studio in Burbank, Calif., is a rat’s nest of old newspapers, geek memorabilia and garbage. Human resources once got involved, but Walton won; the giant troll statue stayed.

So it was a bit disconcerting recently for Walton, 40, to find himself on display in Roy Disney’s former office, a rarefied space built inside a giant version of the sorcerer’s hat from Fantasia. Nervous and fidgety, Walton was giving an interview under SWAT-team surveillance from Disney publicists. The subject: Bolt, the studio’s film about a dog who mistakenly thinks he has superpowers.

Walton is the unlikely voice of Rhino, an overweight, television-obsessed hamster. Described in the script as “rolling thunder” because he is both excitable and confined to a plastic exercise ball, Rhino gets most of the laughs. Walton is so thrilled that he can barely contain himself, but it’s not because an average guy like him is getting more attention than John Travolta, who provides the voice of Bolt.

“Who cares about fame and fortune?” he said, clenching his fists in excitement and waiving them in the air. “I’m going to be a plush animal.”

The computer-animated Bolt, which opened in theaters last week, is the story of a German shepherd who has lived his entire life on the Hollywood set of his own television show, playing a superhero dog who battles evil to protect a little girl named Penny. His days of scripted triumphs come to an end, however, when a studio worker accidentally mails him to New York.

Trying to find his way home, Bolt meets a sarcastic, streetwise cat named Mittens (voiced by Susie Essman of Curb Your Enthusiasm). Still convinced he has superpowers — despite heavy mocking by Mittens — the two set off on a cross-country trek and are soon joined by Rhino. The hamster, so excited to meet his favorite TV star that he fogs up his ball, has memorized every nerdy detail of Bolt’s televised missions.

Director Chris Williams said he auditioned several actors to voice Rhino (he’s not naming names) but couldn’t find anybody who performed the role better than his slightly peculiar colleague. “Mark is emotional and excitable and has a big personality, which is exactly Rhino,” Williams said.

Director Byron Howard added: “All that enthusiasm that you hear in Rhino is Mark’s everyday persona. It’s not put on at all. Every time we’d hear a new recording with him, we’d just crack up.”

It is rare for an animator to land a major role in a big-budget film — Disney executives can’t remember another example at the company, though Brad Bird, the director of the Pixar film The Incredibles, supplied the voice of Edna Mode, that movie’s haughty fashion designer — but employees routinely provide “scratch” voices for characters early in the filmmaking process.

Because each professional voice is recorded separately, and later in the process, directors recruit secretaries and people like Walton to do early readings. Animation directors find it particularly helpful with timing and humor.

Walton had managed to squeak into the credits of two earlier Disney movies by providing a scratch voice that directors decided to keep. He had a cameo in the 2004 dud Home on the Range as Barry and Bob, two longhorn steers. In 2005’s Chicken Little, he got to voice the small role of Goosey Loosey, which involved a lot of honking and squawking. For Rhino, Walton volunteered to provide the scratch voice. It was so good — particularly his high-pitched, high-energy laugh — that the co-directors kept asking him to record more lines as the script changed.

Soon, the studio announced Travolta’s casting. Then Essman’s contract was drawn up. Walton kept waiting for the day when he would arrive at work to find out that Danny DeVito or some other celebrity was taking over for Rhino.

A couple months ago, though, the co-directors asked Walton to meet them in the recording studio. He assumed it was to re-record a line that was slurred or too quiet. Instead he was told he was the voice of Rhino.

The directors suddenly had a very animated 6-foot-2 bearded man on their hands. “I jumped up and down and screamed like a girl,” Walton said. “There’s a videotape to prove it.”

As that videotape attests, Walton is a bit of a character in real life. One year at Halloween he arrived work dressed as the eccentric teenager at the center of Napoleon Dynamite. Williams recalled the time Walton attended a Beastie Boys concert and was dancing so manically that he was asked to calm down. “Which is saying something,” Williams said with wide eyes.

Born and raised in Salt Lake City, Walton originally set out to be a newspaper cartoonist but applied on a whim in 1995 for an internship at Disney’s Florida animation operation, now defunct. There he met Williams, who started at the company as an intern in 1994 and became a story apprentice on Tarzan.

A collector of movie posters, children’s story books and Star Wars figurines — a giant Han Solo stands guard next to that troll in his cubicle — Walton is a fixture at Comic-Con, the annual comic book and movie marketing extravaganza in San Diego.

“I’m a fanboy, I admit it,” he said, adjusting his “H.R. Pufnstuf” T-shirt.

As if to prove his geek credentials, he then broke into one of his lines from Bolt — a sequence that oddly describes recent events in his own life: “Ring, ring. Who is it? Destiny?

“I’ve been expecting your call.”