The first model arrives at 10 a.m. sharp. Her name’s Geidre and she’s pale, has curly hair, and possesses the otherworldly, limby model beauty that’s vaguely praying-mantis-like. She appears to have no breasts or hips; her thigh is about the size of model casting director James Scully’s upper arm. Geidre takes off her flat winter boots, puts on heels, and without a word, walks the length of the room, stops, turns, and walks back. Scully has her stand against a white wall with a bit of pink tape stuck to it. He takes her picture with a Polaroid camera as she stares ahead, expressionless. Then she’s gone; the whole encounter took about three minutes. “The tape is the six-foot marker. I just need to make sure they clear that in heels,” Scully explains. Geidre did, but that’s not enough to make the cut. “I do think that Geidre’s a diamond in the rough, but she’s just not ready yet,” Scully says in a low voice. The next girl stands by.

Scully is a casting director, show producer, a 25-year industry veteran, and one of a handful of prominent model scouts whom high-profile designers trust to assemble the 25 or so girls who will set the clients’ clothes off to perfection. He estimates that he has seen more than 30,000 models in the course of his career. Today—the Wednesday before Fashion Week—he’s working out of Derek Lam’s showroom in Chelsea, putting the finishing touches on the casts of Lam, Jason Wu, Carolina Herrera, Zac Posen, and Bill Blass. A stream of young women go through the same walk-tape-Polaroid ritual every few minutes.

In between arrivals, Scully tries to describe what makes a model right for a show. “For Herrera, I look for girls with richness and poise. They should have a bit of the Park Avenue matron in them,” he says. Derek Lam’s girls are “lighter, sexier, and more sensual.” He likes a model who has confidence in her walk, and a sense of ownership in the room. To the untrained eye, the process may seem amorphous, but Scully makes his decisions in a split second.