The patron saint of actors, St. Genesius, was much on the minds of two dozen hopefuls who had been corralled for a holiday casting call recently in a cramped waiting room outside the Roundabout rehearsal hall on West 45th Street. This was, in fact, the final callback, and it had come down to 12 young men and 12 young women, ranging in age from 12 to 22, vying for 10 roles in “Spring Awakening,” a musical version of Frank Wedekind’s classic play of raging hormones gone tragically amok.

In the vast and under furnished studio that lay on the other side of the waiting-room door, a small creative team – among them director Michael Mayer, producer Tom Hulce, songwriter Duncan Sheik, and author Steven Sater – pored over the contenders’ glossy headshots, sorting through them to see who’d make the cast cut, arranging and rearranging photographs in neat, meaningful clusters. Then, the casting director made his move.

A large man in a black sweater and matching pants with a no-nonsense air about him, James Carnahan, 46, moved to the door and opened the floodgates. In rushed the wannabe stars.

“The casting director is the bridge,” explained Mr. Carnahan, “the only one who moves back and forth between the two rooms, the liaison between auditioners and the creative team. It’s about seeing a lot of people and then bringing the best of them to the director.”

Accordingly, Mr. Mayer then took over, creating his own musical clusters – first groupings, then pairings, and finally solos, with each of the performers allowed to sing a lyric while Mr. Sheik strummed along on the guitar. It was very much Mr. Mayer’s show, but it was deftly stage-managed by Mr. Carnahan, who shuttled the auditioners through these musical hoops and returned them to the holding area while the showbiz heavies reshuffled the 8-by-10s in a new order of preference. Those who rose to the top Mr. Carnahan retrieved for scene readings; the rest he released.

All this for a single staged concert reading February 2 that will be part of the “American Songbook” series. It will be presented in the series’ new home at Jazz at Lincoln Center’s Frederick P. Rose Hall in the Time Warner Center. “Then,” said Mr. Mayer, “we’re going to see where it wants to live, what theater it would work at best.”

“Spring Awakening” was to have debuted three springs ago, but the financial fallout that followed September 11, 2001, limited Roundabout’s budget, and the show has been trying to get back on track ever since. This is the third reading of the show Mr. Carnahan has cast for Mr. Mayer. All told, they have done 10 shows together, including a Tony-winning Best Play (“Side Man”) and a Tony-winning Best Revival (“View From the Bridge”).

And when Mr. Mayer started working in film last year – directing “A Home at the End of the World” – Mr. Carnahan made his debut as well. Interestingly, for this Michael Cunningham story of two boyhood Cleveland kids who renew their friendship as adults in Greenwich Village, he cast the film with an Irishman (Colin Farrell) and three Texans (Dallas Roberts, Robin Wright Penn, and Sissy Spacek), and it played beautifully.

Casting-wise, there’s not much difference between the two media, insisted Mr. Carnahan, but there is in terms of material: “In film, you have more roles and they’re less developed. There’s much more of a sense that you have to cast people who are the part. In theater, where the roles are better written and more developed because there are fewer characters, then you have more time to get beyond that so you can go a little quirkier with the casting.”

Although Mr. Carnahan harbors no directorial aspirations of his own (“When I read a piece of material, I hear what it would sound like, but I don’t see it – I don’t have a visual sense”), he often thinks like a director – this, according to Mr. Mayer: “Jim spends an enormous amount of time getting into the head of the director. He’s always incredibly passionate and falls completely in love with the project when you’re working with him.

“And, on a completely other level, he knows every actor. I don’t know anyone else who works as hard as he does. He sees absolutely every play, every movie, every television show. There’s not an actor out there that he’s unaware of. It’s almost obsessional.”

This year Mr. Carnahan’s obsession has achieved a special magnificence. Of the 16 Broadway shows that have opened during the first half of the 2004-05 season, he has cast five of them, providing parts and employment for 110 actors – “After the Fall” (15), “Twelve Angry Men” (16), “Democracy” (14), “Pacific Overtures” (23), “La Cage aux Folles”(32) – and another 76 follow suit in the three shows he will do after the first of the year: “The Pillowman”(11),”A Streetcar Named Desire” (15), “Chitty Chitty Bang Bang” (50). No other casting director approaches those kinds of numbers this season.

The day after “La Cage” opened, when normal breathing returned and he could have finally taken a break, did he? Of course not. Instead, he took in back-to-back workshops that he had set up in town – a “Threepenny Opera,” for Roundabout, with Alan Cumming, Christine Baranski, Edie Falco, and Wallace Shawn, followed by one of “110 in the Shade” with Audra McDonald, Michael Cerveris, Dick Latessa, and Steven Pasquale – and the day after that, he was off to London to put three American actors (M. Emmett Walsh, Elizabeth Franz, and Lauren Ambrose of “Six Feet Under”) in a production of “Buried Child” over there, and before he left, he’d lined up two shows for next season: the powerfully reviewed “Festen” and Andrew Lloyd Webber’s “The Woman in White.”

How does Mr. Carnahan explain his high tide – accident or timing? “It’s a bit of both,” he offered. “It just all happened. A lot of directors I’ve worked with before just happened to have shows this season. And, too, one of the things that happens with me is, because I have the Roundabout as my base, I automatically have five Broadway shows a season. Some I share with my partner at Roundabout, Mele Nagler, and there are other people in the office who help me with the freelance work that comes in between Roundabout jobs.”

His imposing size seems to argue with the affability and accessibility he displays on both sides of the casting table. Part of this, he conceded, is the Southern sense of decorum that still clings to him years after moving to New York from his childhood homes in Murfreesboro, Tenn. and Atlanta.

“But it’s also having been an agent for 14 years. I heard so many horror stories from actors about being kept waiting forever or about people being cold in the room. It’s a sensitivity born from having been on the other side, having been the guy the actor called after the audition to talk about what a horrible experience it was.”