You may have seen commercials for that new George Clooney movie. The trailer flashes his face along those of Josh Brolin, Scarlett Johansson, Ralph Fiennes, Tilda Swinton, Channing Tatum, Jonah Hill and Frances McDormand, but you might sit there thinking "What is this?"

It's Hail, Caesar! the Coen brothers' love letter to the Golden Age of Hollywood, in theatres Feb. 5. The story follows Eddie Mannix, studio fixer for Capitol Pictures (a.k.a. the Olivia Pope of Tinseltown), as he tries to find out who kidnapped Capitol's highest-grossing star, Baird Whitlock (Clooney). Along the way, we meet the other studio's stars, and what is a 1950s backlot without a Gene Kelly?

Enter Burt Gurney (Tatum), Capitol's song-and-dance man — and the reason every theatre lover will want to see this film. The actor who got down and dirty in his choreography for "Magic Mike" now laces up his tap shoes to revive the appearance elaborate tap numbers in the pictures with a six-minute sailor routine called "No Dames!"

Only problem: Tatum didn't tap. According to the movie's choreographer, none other than Tony winner Christopher Gattelli, when Tatum signed on, the original script alluded to a simple Gene Kelley-like dance on a boat. "It was literally a few sentences because it was going to be a blip that then cut into a scene," says Gattelli. "Then the Coens saw a clip of Donald O'Connor tapping, and they were like 'Oh, we want Channing to tap, that feels really of that period,' and I was like, 'Does he tap?' and they were like 'We don't know, but he's got to do it!'"

Tatum was on board. "Channing was like, 'Yes! I want to do this. That's what the Coen brothers want.'"

Gattelli was thrilled to lend his tapping chops. "My inner kid just busted because I grew up watching all of those classics, every Gene Kelly movie, every Fred Astaire movie," says Gattelli. "It's definitely one of my favorite forms. It was the thing I started doing first … when I was eight."

After the Coens officially hired him in fall 2014, Gattelli assembled his group of seamen. "The whole thing was about protecting Channing because he is such a great guy and it was a lot to take on for him," says Gattelli. Gattelli — who also appears as the choreographer in the film — recruited his associate Lou Castro and mainly cast his dancers from the pool of men he'd worked with, including former Newsies Ryan Breslin and Evan Kasprzak and other seasoned Broadway ensemblists like Tyler Hanes, Casey Garvin, Adam Perry and Ryan VanDenBoom. "To have that kind of energy and support in a room, it's the best."

The choreography demanded Tatum cram ten years of tap training into two months before a month of shooting. "He was like, 'I'm going to get that, don't dumb it down, I want to do that,'" says Gattelli of Tatum's reaction to seeing the routine for the first time. "It takes a special person to never have done this, and you're seeing these guys who've done this their whole life… He wasn't put off by it or scared about it."

On the contrary. Tatum threw himself into rehearsal. He began with private lessons with an assistant to Gattelli, Meredith Patterson, in November 2014. Gatelli flew out to Los Angeles to train Tatum beginning in December. "[Channing] was rehearsing his final shot in the movie, and in between every take, and during all of his lunch breaks, his dinner breaks — they had rented a separate soundstage just for Channing to practice," says Gattelli.

At the time, Tatum was also flying back and forth to Vegas to shoot Quentin Tarantino's "The Hateful Eight." "He was practicing and bringing his tap shoes on set to Quentin's movie," Gattelli marvels. "He was relentless."

Tatum told Vulture in an interview that Gattelli's belief in him kept him going. "He believed in me until the day I was about to crack," Tatum says. "I didn’t think tap-dancing was going to be easy, but I also had no idea how hard it was actually going to be. … you actually have to be musical. I’m used to dancing on the beat, or riding the beat, and with this you are the beat."

As for that day Tatum felt like he would crack, Gattelli remembers it well. "That's that day I [kept saying] 'Again… Again… Again.' It was like a personal trainer [because] he was like, 'Push me,'" Gatelli recounts. "I was like, 'Ok. Again. Eight counts. Again, let's break it down to four counts. Four counts, go. Again. Again.' … I think he just wanted it so bad and I knew — I could tell — it's that breaking point for anyone where he knew he was so close."

After a month of shooting in January 2015, Tatum wasn't close, he landed in the pocket. "Watching his determination…" says Gattelli, "It was a really beautiful thing to see."

Ruthie Fierberg is the Features Editor at Playbill.com. She has also written for Backstage, Parents and American Baby, including dozens of interviews with celeb moms and dads for parents.com. Follow her on Twitter at @RuthiesATrain.