Jesse McCartney disappeared just as his career was cresting. It turned out to be a brilliant move — as a result, the singer, songwriter and actor is now having one of the biggest years of his career.

“Leavin’,” the first single from his third album, “Departure,” was No. 1 on the top 40 charts for six weeks, selling more than 1 million digital singles. He co-wrote the song of the summer, Leona Lewis’ “Bleeding Love”; voiced toon characters in “Alvin and the Chipmunks” and “Dr. Seuss’ Horton Hears a Who!”; and saw his feature acting debut, in the indie film “Keith,” finally hit the bigscreen (after shooting it three years earlier).

Though he may have been MIA, McCartney was far from idle. “I wanted to go away for a couple of years and hide and be creative,” he says. “A lot of that was writing in the studio nonstop with other writers and by myself, taking trips to different parts of the world to write. When I came back on the scene, I wanted to have something to show for it.”

One of those songwriting sessions gave McCartney what he’d been looking for: credibility. McCartney and Ryan Tedder of OneRepublic wrote “Bleeding Love” and felt it would be a good fit for a female vocalist. Simon Cowell and Clive Davis agreed, giving the song to Lewis, Cowell’s “X Factor” winner. “Bleeding Love” has hit No. 1 in 32 countries.

“We had no idea it was going to be the hit it was,” McCartney says.

The success also taught McCartney to be more concerned with trying than with failing: “I’m learning, at age 21, you can really do what you want, so I’m thinking of new ways of entertaining. I want to put together a production company and start producing. If it doesn’t work out, it doesn’t work out.”

Recent breakthrough: “A reinvention of my whole music career sonically. I’ve broken down a bunch of walls and reached past many hurdles to show growth to my fans.”

Role model: “Will Smith. ‘Big Willie Style’ is one of the first records I bought as a kid, and now he’s the biggest movie star.”

What’s next: “I just released my second single. The plan by early next year is to start touring and build up my acting repertoire.”