The difference might come down to choosing the right songs.

Susan Slamer and Debra Byrd know that. As the "Idol" music director and vocal coach, respectively, they've seen extremes.

A high: Last year, Fantasia Barrino chose "Summertime" — a song she'd never heard of, written 50 years before she was born.

"She said, 'I chose it because I feel it ... and I know exactly how I'm going to do it. I'm going to sit on the floor, barefoot,' " Byrd recalls. Barrino drew raves and was soon the champion.

A low: This year, singers could choose any song from the year of their births. Nadia Turner chose "When I Dream," an obscure 1977 song.

"Nadia is a very confident young woman," Slamer says. "She did a song that no one had heard of."

Byrd recalls that. "She said, 'I'm going to choose a Crystal Gayle song.' I said (sounding doubtful), 'Okaaaay.' I even said, 'Is this your final answer?' "

It was. Judges grumbled and viewers failed to vote to keep her on.

Such choices start anew each week. New categories are announced; the decisions begin.

"Susan Slamer has a listening party," Byrd says. "She plays hundreds of songs for them."

While others in the first season were groaning about big-band week, Kelly Clarkson instantly announced she was doing a song from the Bette Midler movie "For the Boys." While others in the current season floundered during Broadway week, Constantine Maroulis was at ease — he's a musical-theater veteran.

At times, definitions are disputed. During dance-song week, producer Nigel Lythgoe argued that Bice's choice ("Vehicle," by The Ides of March) didn't fit the category. "I told him, 'Excuse me, but in my neighborhood we danced our butts off to that song,'" Byrd said. "Maybe they didn't in Nigel's neighborhood, but they did in mine."

Some weeks are naturals for the young singers. Barry Manilow's songs, with their direct melodies, went smoothly. Elton John's complex ones created a rough week.

One person, however, conquered a John song. "Jennifer Hudson had sung 'Circle of Life' hundreds of times," Byrd says. "She came in with an arrangement and a tape recording."

That repetition helps. Bice does songs he's been doing in rock clubs for years while others sometimes struggle to learn them in a week.

Each singer faces different challenges, Byrd says. "For Carrie (Underwood), it's personality. For Anthony (Fedorov), it was focus."

Underwood is a country-style singer, content to stand and emote. She began learning and expanding. When she growled "I'm evil" while kicking over a microphone stand, the crowd was delighted.

Vonzell Solomon has needed no nudging. "Vonzell just oozes personality," Byrd says.

She's a natural on stage and off, Slamer says. "She's always smiling, always energetic."

As Underwood stretches for that same kind of energy, she faces complications.

"Piece of My Heart" was a blistering Janis Joplin rock song in 1968, then a Faith Hill country song in 1994. Underwood tackled it. "She listened to the Faith Hill version," Byrd says. "That's the one she aligned with."