For the fiercely contested "Dancing with the Stars" semi-finals, its Emmy-winning costume designer Randall Christensen had his own workroom competition: Create 12 distinct looks for four couples in five days.

His team's challenge included creating a bedazzled neck brace for injured professional Kym Johnson while transforming Karina Smirnoff into a cat woman and giving shrinking actress Kirstie Alley her sexiest costume yet.

With each couple dancing three dances the design process was "an enormous challenge," says Christensen. "Our saving grace was that we had to deal with only four couples instead of the full cast and we know their personalities and body types."

Here, Christensen takes us behind-the-seams to dish on some of the night's best looks:

Photos: Adam Taylor, ABC

Kym Johnson and Hines Ward: Up until moments before they took the dance-floor for an emotional Argentine tango that received a perfect score, Christensen's team was working on a silver-crystal encrusted neck brace for an injured Johnson. She was rushed to the hospital May 13 when football pro Ward's full body weight came down on her neck as the couple attempted a difficult flip.

Johnson ultimately made the call that she felt strong enough to dance without the brace. Her formfitting tango dress with its cutout back and angled hemline was the most labor intensive costume of the night featuring mesh encrusted with "thousands upon thousands of pearls." "Then we added the sparkle of 10,000 crystals wedged between the pearls," says Christensen. "To make that costume in five days was a huge undertaking."

Karina Smirnoff and Ralph Macchio: For their pimp-daddy inspired salsa, Christensen created a skintight bodysuit of leopard for Smirnoff, splitting it down the middle to reveal a red crystal-encrusted bra. "Who else could pull off such a sizzling catsuit but Karina Smirnoff," says Christensen.

Padding obtained from Christensen's "secret downtown [Los Angeles] source" was slipped into Macchio's pants to give him a more prominent backside, an idea the actor and his partner came up with "for fun." The catsuit was accented with thousands of hand-strung crystal strands and piles of hematite and topaz crystals.

Photos: Adam Taylor, ABC

Kirstie Alley and Maksim Chmerkovskiy: Christensen says Alley's transformation (she says she's lost more than 70 pounds dancing) "blows me away each and every week." And this week, he showed off her toned upper body in a sleeveless, strapless frock. Her dark paso doble look featured a crystal-encrusted bustier with a tattered black and purple organza neckline. "The draped front was not traditional, but kept with the theme of the dance, which is Spanish gypsy," he explains. Black leggings were added in keeping with the edgy mood.

Chelsea Kane and Mark Ballas: The Disney star rose to the top of the leader board again, thanks to her sizzling rumba which she performed in a deep mauve and lavender, midriff-baring frock that had its own dramatic backstory. The costume was originally cotton-candy pink, but the dancers asked for a new, darker look after show producers informed them clearance issues had forced a last-minute music change. "Being the pros that they are and knowing the kind of week the workroom was having, they were cool with my idea of dyeing her original look. We just dipped the whole thing and said a few prayers."

For more behind-the-seams "Dancing with the Stars" fashion scoop read our last recap.