If you slipped into the theater right before tonight’s curtain went up, you missed one of the Fox Theatre’s greatest attractions: Organist in Residence Larry-Douglas Embury, who plays pre-show concerts on the theater’s 1929 Möller pipe organ, “Mighty Mo.”

Prior to his appointment as resident organist in 2001, Embury says Mighty Mo only was played for summer film series sing-alongs. Now, he says, “I open for the majority of the shows. … I end 15 minutes before the show starts, so the orchestra has time to tune up, and I usually start when the house opens.”

Embury prefers not to use a set list of songs for his concerts. Instead, he likes to spotlight music from upcoming shows and play audience favorites. “We have almost 4,000 volunteer ushers, and after nine years, [I’ve] got to know a lot of them. One likes to hear ‘Besame Mucho’ and one likes to have me play ‘Hello, Dolly!’ And I take suggestions from people who come by the rail [separating the organ from the audience]. One night a lady came by and [asked for] ‘Claire de Lune.’ It’s one of my favorites, so, I played it next. I don’t have any agenda; it’s all about them.”

One of the joys of what Embury calls “having the best job in the world” is getting to play such a rare, world-class instrument. “Mighty Mo has all the bells and whistles, and can handle any musical literature I throw at it, from Bach to boogie-woogie, rock ’n’ roll, Broadway, gospel, hymns, country-western and scores for ballet.”

Part of what makes the instrument so flexible is that its keyboard and stops are connected to a whole gallery of musical instruments hidden behind the faux opera boxes on either side of the stage. Mighty Mo’s catalogue of sounds range from a 1929 Ludwig drum kit, Zildjan crash cymbal and grand piano to carillon bells, a real marimba and a locomotive whistle. “They’re under wind pressure,” Embury explains. “So when I activate, say, the castanets, if you were in the chamber, you’d see the castanets look just like they were in a dancer’s hands, but they’re under wind pressure [which makes them] ‘speak.'” Embury says that when he plays, he must think like an arranger, deciding whether and when to introduce brass instruments, strings, percussion and sound effects into the musical score of each song he plays.

And he loves surprising the audience with songs they wouldn’t expect to hear from such an august instrument. For last year’s Coca-Cola Summer Film Festival, the Robert Downey Jr. movie Iron Man was scheduled, so Embury learned how to play the Ozzy Ozborne/Black Sabbath song “Iron Man” on the organ. “When I played it, the audience was up for grabs,” he laughs. “But that’s what it’s all about. We have an intergenerational audience — we have grandparents come to see the shows with their grandkids and everything in between.”

Perhaps what Embury enjoys most is playing for the children who come to the Fox. “The first time I ever saw a theater organ was in the Orpheum Theatre in Vancouver … when I was a little kid of five, and it really did something to me. So I think, ‘There’s a little Larry standing there,’ when I play and see their eyes as big as saucers. It’s exciting for adults to see that big behemoth [Mighty Mo] rising up out of the orchestra pit, so I know it must be exciting for little kids to see it.”

If children (or adults) are interested in learning how to play the theater organ, Embury says there are two wonderful organizations with chapters in Atlanta they should contact: The American Theatre Organ Society and the American Guild of Organists. “[They] have scholarship programs for kids so they can … study with teachers, so when I go to that great keyboard in the sky, there’ll be someone to come after me,” Embury chuckles.