On a crisp November night in 1966, the would-be prince of New York City threw a ball that even Cinderella would have had trouble getting into. The prince was Truman Capote, and the party was his Black and White Ball, honoring the Washington Post’s publisher, Katharine Graham. Around 540 handpicked guests turned up to the Plaza Hotel for what Capote would describe as “a little masked ball” featuring New York’s finest, from Lauren Bacall to Norman Mailer to, of course, his beloved swans. Capote’s Black and White Ball wasn’t just the event of the season—it was the society event of the ’60s.

Naturally, FX’s Feud: Capote vs. the Swans devotes an entire episode to the event—from its meticulous planning to the night of the picture-perfect party. To bring the Black and White Ball to life, executive producer Ryan Murphy and director Gus Van Sant enlisted famed fashion designer Zac Posen. “Gus is a good friend, and Ryan is a friend and somebody I’ve known for a long time. We’ve wanted to work together for a long time,” Posen says. When the team asked the designer if he might want to fabricate the episode’s elaborate gowns, they were in luck: “I don’t have my own line anymore,” he says. “So I only do one-of-a-kind pieces.” (Days after our interview, Posen would be named the executive vice president, creative director of Gap, and the chief creative officer of Old Navy.)

Running a major fashion label is not the same thing as costuming a major television show. “Costume design is a different consideration—more character building,” he says. “It’s also elevated. Not only is it costume design of a time period, it’s costume design within the wonderful world of Ryan Murphy, aesthetically set between Ryan and his collaborator [Feud costume designer] Lou [Eyrich], of a world that they’ve created through the lens of Gus Van Sant. So you kind of find your place within that.”

But Posen didn’t feel obligated to hew entirely to the historical record. In fact, Murphy specifically told him not to. Still, he did his homework. “I was finding historical matches of fabrics, of color sources, deadstock flowers from one of the oldest silk flower places in New York of that time period,” Posen says. He worked closely with Deborah Davis, author of Party of the Century: The Fabulous Story of Truman Capote and His Black and White Ball. Then Posen combined his research with his own creative vision. The result? A true feast for the eyes.

Every dress, Posen says, “is a character—a different, beautiful, aquatic bird on a pond.” Posen was inspired by the period’s magical surrealist filmmakers like Federico Fellini, as well as Franco Zeffirelli, British photographer Cecil Beaton, director Vincente Minnelli, and even Muppets creators Jim Henson and Frank Oz. “I was like, Let’s look at the people that I think are great imagineers in filmmaking, that are a really big part of my DNA,” says Posen. “How do we create that kind of magic [when] it’s going to be in a piece that takes place around historical characters, but is not a retelling of history?”

All the research in the world wouldn’t have made a perfect recreation of the ball possible. “Slim [Keith]—you can’t find documentation of what she wore,” says Posen. There were “no drawings or written descriptions that I could find, or any of the historians could find.” In other cases, there was too much history to pore through. “Lee Radziwill had the time of her life that night,” he says. “There’s a million Slim Aarons photos to Ron Galella photos and drawings [of her]. It’s all there, documented.”

There were practical matters to contend with as well. Directed by Van Sant, the episode “Masquerade 1966” was conceived as a movie within a show, shot mostly in black and white from the perspective of the Maysles brothers—the documentarians behind Grey Gardens, who were working on a documentary about Capote around this time. According to Posen, it was decided only partway through the process to shoot the episode almost entirely in black and white. Naturally, this had an effect on how Posen chose to costume certain swans.

Take Radziwill (Calista Flockhart), for instance. In real life, the socialite wore an embroidered Mila Schön dress to the ball, but Posen didn’t have time to embroider a dress. “I got the briefing right before Thanksgiving, and we shot in the beginning of the second week of January,” he says. “It was quick.” Flockhart was initially in a white jacquard cloque coat, which Posen says looked great on her, but it was “just going to disappear” on camera. So he pivoted: “Let’s use a metallic jacquard, let’s do an overlay of something graphic with these embroidered daisies, then trim it with something that looked kind of [like] futurism.”

For Slim Keith (Diane Lane), Posen was able to tap into his imagination and design an outfit that both felt historically accurate and could also tell her character’s story. “I wanted to have her come in with hot air, you know?” he says. “A little bit of ‘the wicked witch swooping in’ quality to it.” Thus Slim’s cloak resembles a hot-air balloon of fabric. Why does Posen believe Keith would enter the ball that way? “She wasn’t asked to be the host. I think she was probably really upset,” says Posen. “In history, she definitely went through a side entrance to the ball.”

While Keith’s cape tells an emotional story, the rest of her outfit hews closely to her known personality. “Slim Keith, for me historically, always wore pants…and white shirts,” says Posen. “She was ahead of her [time]. I thought those were real style moments of hers. So I gave her these large palazzo pants, tuxedo, cummerbund, and then kind of a white kimono top.” With Keith, Posen couldn’t resist “the things that create movie magic—the inflation of air and then that black-and-white silk taffeta, bringing the drama.”

Posen also couldn’t resist bringing the drama himself. The designer makes a blink-and-you miss-it cameo in the episode as Slim Keith’s escort to the ball. “I catch the cape and sweep it around, so that’s something to spot,” he says. “I had a fedora mask on. It was so much fun.”

When it came to C.Z. Guest (Chloë Sevigny), Posen wanted to stay true to her blue-blood aesthetic while tapping into her wilder side. “She’s the prim and proper one, but then C.Z. famously was also a showgirl in her youth,” the designer says. To capture that dichotomy, he referenced the few photos of C.Z. from the event and put Sevigny in an “American couture, strapless, fitted dress” more reminiscent of a performer. “The back of her dress has this elongated draped back and silk flowers on the top of it that hark to like a large horse ribbon, because C.Z. was a big equestrian,” says Posen. “I think Truman described C.Z. as like cold milk, right? Or cold cream. She’s cold cream.” (Capote reportedly described her as a “cool vanilla lady.”)

“I love that description. I wanted it to look like you put your finger in cold cream and made a shape of a swan in the drape of it.”

Posen says Joanne Carson (Molly Ringwald) got sick and couldn’t attend the Black and White Ball, but in Feud’s alternate reality, Carson made it to the fête. Like a good student, Posen says he found the outfit Carson actually would have worn to the ball—a black column dress with a Pierre Cardin necklace. “It wasn’t going to be enough for TV,” he says. Instead, Posen wanted to lean into Carson’s woo-woo, midcentury, California energy. “I love that she’s wearing two different color gloves,” says Posen. “And then she wore a real Alexander Calder art necklace. It was a moment of modernism.”

In Feud’s version of events, Ann Woodward (Demi Moore) crashes the ball in spectacular fashion, sporting an elaborately bejeweled and feathered mask which she removes to engage in another epic head-to-head with Tom Hollander’s Capote. Outfitting Moore for this scene was a particular delight for Posen, given their closeness. “Demi is really like my sister,” he says. “It’s such an exciting moment for her. I wanted her to have this big reveal—this Russian net with crystals embroidered on it. And then have this plumage and silk tulle flowers on the side, almost as if it was a helmet—like Artemis, a Roman helmet in a way.”

The headpiece’s allusion to armor is fitting, given the war waging between Woodward and Capote. Its see-through nature captures the vulnerability Woodward exposes in her losing battle at the Plaza. “She had to put on this kind of disguise to enter, but also something that would show the fragility of the moment,” says Posen. “The shape of the crystals and the feather—it would show her bravery and her fear when Truman confronts her. It has wings, which is very much in the time period. It’s kind of similar to Audrey [Hepburn] dresses in that time period, but also a little bit of what Marella Agnelli wore, or dresses that Gloria Guinness—and other swans that are not in this story that were in Truman’s world—would have worn.”

Woodward’s outfit was not the only one to reference birds. Posen pored over photos of Babe Paley (Naomi Watts) from that evening. “We couldn’t use fur. It was Disney, and it didn’t seem interesting to me,” said Posen, of Babe’s actual outfit. What’s more, Paley’s actual dress paled in comparison with his fantasy. “It wasn’t enough. Babe as queen swan needed a grand entrance.”

So he decided to swap fur for feathers. “I started looking at the construction of an actual swan wing—how it would move, and how it could open as two wings. I just kept building it and building it, and shaping it so it almost looked like a swan sleeping.” Lined with marabou feathers, Babe’s swan coat really does resemble a slumbering bird when it’s closed.

But when she opens it, the real fun begins. “I think that there’s a real reveal moment,” says Posen. “The collar had a lot of swag to it and theatrical surprise. It’s definitely more than Mrs. Paley probably would have ever worn in reality. But this is about having that moment, and that’s the great part about storytelling.”

There’s always a black swan in the bunch. In the only moments from the episode that appear in color, Jessica Lange reenters as the ghost of Lillie Mae Faulk, Capote’s mother. Free from any historical references, Posen went all in on turning the Oscar winner into a dark apparition.

As with Slim Keith, Posen wanted to tell Lillie’s life story with her outfit. “Truman’s mom was very obsessed with fashion and style—getting into society from the South, and this journey of what she had to go through to get there,” he says. He encapsulated that in a timeless look that referenced the 1910s through the ’60s, “as if you went to a vintage shop and put all of these elements together to create this black swan look.” The dress had “a 1920s/’60s shape with the marabou trim. All the plumage is Victorian or ’30s.”

Posen touched on the ’50s with Lange’s taffeta-covered arms, the ’20s with the sequining, and “the body of it had these black gelatin sequins and crystals,” bringing a ’40s element. “That was such a crazy, wonderful, magical moment to see her in action.” Why, though, is it black? The dress, he says, “has been blackened by her anger at her position.”

The dress was meant to match the “extravagance of a waltz” that serves as the climax of the episode, in which Truman—even at one of the crowning events of his life, is unable to fully be present due to the demons of his past. Or as Posen succinctly puts it, “It’s always about the mother.”

Juggling history with the demands of a big TV production may seem more creatively constricting than designing a gown for a regular old red carpet, but Posen feels quite the opposite. “In terms of how I built fashion versus this, I mean, my creative process in some sense felt more freeing in this process,” he says. “Way more freeing and impromptu.”

At the end of the day, his goal was to service both the story and the women who play the swans. “These incredible actors and performers, who are also long-time collaborators of mine, they are the subject. But they’re not necessarily the finished client,” he says. “They have to be taken into huge consideration because I’m a collaborator, I’m a gentleman, and I want them to be happy in the process.” And unlike Capote, he’s smart enough not to pick a favorite swan of all. “No, no, no,” Posen says with a smile. “I like my full menagerie.”