During his years as the star of the TV series “Knight Rider,” David Hasselhoff was asked to sing at Jerry Lewis’ annual Labor Day telethon.

He didn’t get past the rehearsal.

“That was my worst moment, being on stage at rehearsal and not being prepared,” Hasselhoff recalls in a recent phone interview.

“I was just not ready. I was singing ahead of the orchestra, it was incredibly embarrassing. My musical arranger went over to the conductor afterwards and said, “Thanks for the learning experience.”‘

Hasselhoff eventually appeared on the telethon, but not as a singer. So he knows from flop sweat.

And he’ll see his share of flops and sweats when he returns as a judge for the third season of “America’s Got Talent,” premiering at 9 p.m. Tuesday, June 17, on NBC.

Hosted by Jerry Springer, with Sharon Osbourne and Piers Morgan joining Hasselhoff on the court of appeal, “America’s Got Talent” differs from, say, “American Idol” or dancing competitions because it’s open to a wider variety of acts – not just singers and dancers, but ventriloquists (like last year’s winner, Terry Fator), impressionists, jugglers, magicians, contortionists, you name it.

In advance of the third season, Hasselhoff traveled the country to be part of regional auditions in Dallas, Atlanta, Los Angeles and New York City, and he’s seen it all, he says.

“Anything goes, from a transsexual Britney Spears to an opera singer to a little girl who performs like a Shirley Temple. Some of it is staggeringly unreal. I sit there in my chair waiting to see what’s coming around the corner next. People will do anything to get on television. But then all of a sudden we’ll have this amazing glut of talent. We can only see so many bad acts.”

Hasselhoff can be of two minds when it comes to TV competitions. On the one hand, he offers that “I wish there was a show like this when I was growing up to showcase my talent.” Then in the same conversation, he’ll go the other way, saying, “I liked making it the hard way.”

However he made it, he made it, certainly in terms of worldwide recognition, or recognizability, from his days as Snapper Foster on “The Young and the Restless” to “Knight Rider” to “Baywatch” to his singing career to Broadway to, well, “America’s Got Talent.”

Hasselhoff, who turns 56 later this summer, has also shown up in his share of tabloids, for broken marriages, one to actress Pamela Bach, who appeared in a number of “Baywatch” episodes, custody battles and allegations of public drunkenness.

“It’s like the “Blair Witch Project’ out there,” he says. “Wherever you go, there’s somebody watching you. There’s no privacy.”

But privacy is what the contestants on “America’s Got Talent” want less of. Hasselhoff understands that impulse, too, though he says that he’s quick to tell his own children, his two daughters, that there aren’t any shortcuts.

“I see all these mothers pushing their kids out on stage, and when the kids would lose you get this attitude,” he says. “I work very hard with my own children to help them keep a sense of reality.

“To make it in this business is not so much about getting on television or getting your picture in the paper or doing as many stupid things as you can, but it’s about working hard. I tell them, “I would be doing you a disservice if I put you out there and you made a fool of yourself because you weren’t ready.”‘

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