Do you know who can sing any type of song, no matter what the style? Back-up singers, which might explain why most of this season's "Idol" favorites seem so devoid of personality.

This week's celebrity guest was famed musical composer Andrew Lloyd Webber, who greeted the contestants and attempted to give them advice on how to tackle his challenging catalog of compositions better suited for Broadway than a TV talent show.

First up was Syesha Mercado, who chose the tune "One Rock And Roll Too Many" from "Starlight Express." She began the song standing on a piano, dressed in a tight, slinky red dress. It was a good choice for her, since it was a tune that isn't overly familiar to audiences. In recent weeks, she's opted to tackle very familiar tunes made by famous by the likes of Whitney Houston and Fantasia. Those comparisions haven't been kind, and she needed a breakout performance to stick around another week. The judges loved the performance, with Randy Jackson telling her it was her best performance ever and Simon Cowell admitting that it was "very sexy."

If Mercado chose a less familiar tune, Jason Castro took the other approach by choosing the uber-familiar tune "Memory." Andrew Lloyd Webber laughingly admitted that he never thought he'd ever see a man sing the song wearing dreadlocks, and it did seem to be a somewhat odd choice. But given Castro's admission that he had never seen a Webber musical, going with a hit must have seemed to be a good bet. But Webber's explanation of the song to Castro seemed to confuse him "He understood it…I think," laughed Webber. Castro then admitted that he had no idea "a cat sang the song."

Weirdly enough, the resulting performance was surprisingly solid. While the song was written for an old character, Castro's breathy singing brought an interesting vulnerability to the tune. While Paula thought he did a great job, both Randy and Simon hated it. With Simon describing his performance as being "somewhat like a young man being forced by his mom and dad to sing a song he hated at a wedding."

Brooke White got the treat of singing a newer Andrew Lloyd Webber/Tim composition. "You Must Love Me" was written for the "Evita" movie. Webber said that he realized she initially didn't understand the song at all, but in the end, he proclaimed she was a "natural born actress."

But her performance quickly went wrong. She began her performance seated on the edge of the stage, and about two lines in forgot the words. She stopped in mid-sentence, muttered that she was sorry, and asked the band to begin again. The problem seemed to throw her a bit, and she continued to struggle throughout the performance. It was almost as if she then concentrated so hard on remembering the lyrics that she forgot everything else.

That having been said, I could see why Webber thought she was a strong actress. She did bring an actual attitude and feeling to the performance, which is often lacking in "Idol" shows. It ultimately was only an "okay" performance, which probably puts her in the bottom three this week. While Randy and Simon were kind (if not excited) about her performance, Paula Abdul seemed offended by the way White stopped and restarted her performance, berating her several times over the incident. Once again, Abdul picks the oddest times to develop a spine.

David Archuleta continued his trend of picking quirky tunes and then reworking them. His choice this week was "Think Of Me" is from "Phantom Of The Opera," and Webber said the song was written for a female diva. Archuleta gave a very solid performance, although he once again sounded a bit like a refugee from a boy band. Randy and Paula both loved the performance, but Simon wisely thought it was "forgettable."

Carly Smithson initially chose another "Phantom" tune, "All I Ask Of You." Webber listened to a part of her rehearsal, then stopped and asked her if she would sing her second song choice, "Superstar" from "Jesus Christ Superstar." While that choice seemed smart in theory-since it's a song that requires a strong set of pipes to pull off-Smithson couldn't quite seem to get the feel of the phrasing down. If you're not familiar with the tune, listen to the classic Yvonne Elliman version from the original theatrical soundtrack. There is no comparison between the two, but despite my dislike of her performance, the judges thought it was all just wonderful.

The final performance of the night came from David Cook, who chose "Music Of The Night" from "Phantom." In his interview segment, he mentioned that he loved the song, and had sung this type of music before. That comfort showed in a performance that blew the roof off the place. Cook traded in his typical rock voice for a more melodic, musical take on the tune. He hit some impressive notes, and it was easily the best performance of the night.

It's always difficult to predict a final three when it gets down to the final six performers. But my guess would be Jason Castro, Brooke White and perhaps Carly Smithson. Although Archuleta deserves to be in the mix as well, given his last couple of performances.