LOS ANGELES - Originally, the concept of inviting music industry veterans onto Fox's American Idol as guests, mentors and judges was simple: Flattery will get one everywhere.

"We really wanted to clear songs. We really wanted to open up catalogs," Idol executive producer and president of 19 Television Nigel Lythgoe explains. "What better way than choosing great songwriters and saying, 'Hey, we want to celebrate your music.'

But somewhere between Barry Manilow and Smokey Robinson, Lythgoe had a revelation: He learned that future Season 3 winner Fantasia Barrino had never heard of the Porgy & Bess classic Summertime and decided there was a bigger picture.

"We suddenly realized we were opening up American music again to America because it's been such a niche on radio and television," Lythgoe says.

In the process of crafting new stars, Idol has become a haven for old ones.

For the past five seasons (Season 1 was guest-free), music industry veterans have appeared on the show as judges, as celebrants in an evening of their hits and as mentors. More than 35 guests - many of whom haven't landed airtime on broadcast television or mainstream radio in years - have found temporary residence on television's most popular show, before a larger audience than most have ever had.

After all, when was the last time Olivia Newton-John appeared in prime time? The Australian actress-singer (who, by the way, won a talent contest as a teenager by singing Summertime) had her songs featured in Season 2, then returned as a judge in the audition phase this year.

She supports picking out older songs for young voices: "You really have to sing well to be able to sing them because they're quite difficult. You have to have really good pitch, and you can't cheat."

Songwriter Diane Warren, who has composed songs with nearly every Idol winner, also sat in on Season 2 as a judge while the night competition featured her songs. But critiquing was a challenge. "

It was a night of my songs, watched by 80 million people or something," she recalls. "I had a hard time being harsh."

As for harsh, Warren warns that judge Simon Cowell's notorious brutality is only a taste of what the real industry holds.

"If you can't take someone being in your face and honest, do not be in the music business," she says. "It's like Simon times a zillion. He's a day at the beach."

The end of Season 5, which featured Prince and Mary J. Blige, proves there's room to get bigger and better with the Idol guest list.

(Sources indicate that the show is trying to book Paul McCartney, who Lythgoe says would be "a dream pick," but Season 6's confirmed guests do include heavy-hitters Tony Bennett, Jennifer Lopez, Diana Ross and Gwen Stefani.

However, there's little doubt that Lythgoe is delighted by the success of his "other" winners: his visitors.

"It's such a big country, and sometimes information doesn't get passed around - certainly music information - very well," he says. "Fingers crossed, we are doing that. Just the delight of seeing peoples' faces light up when Fantasia sings Summertime - we take a great pride in (knowing) we introduced that to them."