LOS ANGELES – Three hours until showtime. The “Dancing With the Stars” ballroom is empty except for the hosts, dancers and band.

The pro dancers and their celeb partners – already in costumes and makeup – have been rehearsing routines for almost a week. The 18-member orchestra, on the other hand, just saw the night’s music and in just a few hours, they’ll be playing it in front of millions.

On today’s playlist? Lady Gaga, Louis Armstrong, the Bangles, Cole Porter, a song in Spanish and a show tune from “Chicago.”

“We don’t rehearse until (show day),” music director Harold Wheeler says. “That’s why it’s a great orchestra.”

Wheeler, a Broadway veteran, has been playing with the same group of musicians for more than a decade. The composer-conductor brings them along on his various gigs, which include film and TV soundtracks and, this year, performing at the Academy Awards.

“They’re on many, many recordings everywhere,” says, Wheeler, 66, of his band, “but on Mondays and Tuesdays, they’re mine.”

The show’s producers pick the songs the pro dancers and their celebrity partners dance to each week.

“A lot of the hip-hop stuff and so forth is to try and draw those younger people,” Wheeler says. But there’s nothing – not even the rare Portuguese, country or “hillbilly” tracks – the band can’t handle.

The dancers approve the songs Monday. Wheeler gets the track list Tuesday and arranges the tunes for his orchestra, giving each what he calls “the ‘Dancing With the Stars’ sound.” Basically, he makes them bigger, tapping into the shiny brass power of the horn section.

He assigns his singers their parts on Tuesdays, too, so they can listen to the originals for almost a week before singing them live.

“Their job is much more difficult, because they have to really be chameleons,” Wheeler says. “When we do a Ray Charles number, they sound like Ray Charles. We do a Lady Gaga number, they sound like her. They’re so wonderfully versatile.”

“Dancing” executive producer Conrad Green says, “They’re fantastic professionals. Every week, I’m continually amazed at how they’re able to turn these things around so quickly. It’s awe-inspiring.”

Singers on “Dancing With the Stars” get to rehearse for an extra hour with Wheeler before the band comes in. Then the whole group runs through each of the night’s songs together four times before the big live show.

“We never really get to digest the songs because it’s our first time with them,” singer Carmen Carter said. “The first time I’m really singing it is on that Monday. That’s the first time you’re hearing yourself on the mike.”

The main challenge is replicating the sound of the original tracks the dancers have been using for practice – especially because the dancers and band spend just 10 minutes together before showtime.

“Sometimes, the tempo can be a little different from what you’ve heard with the recording you’ve been listening to,” says dancing pro Kym Johnson, who won the show’s mirror ball trophy last season with partner Donny Osmond. “But all in all, I think performing to a live band is much better than to a recording.”

Despite the last-minute nature of the show’s musical preparations, Wheeler says the band has only gone wrong once in six years and nearly 1,000 songs.

“Half the orchestra misinterpreted a cue and the band was playing two different things for about eight seconds,” he says, but he’s pretty sure no one noticed.

Green says the big sound of Wheeler’s orchestra “makes the show feel special.”

“It gives it a real sense of occasion and uniqueness,” he says. “You’ll never hear those songs that way again.”