With Brad Pitt and Marion Cotillard as your stars, why not make their costumes seductive?

Elegant and impeccably groomed, Marianne Beauséjour glides into the ballroom, her soft green satin gown instantly identifying her as a woman abundantly aware of her power and charisma.

The floor-length gown commands a critical scene in “Allied,” a World War II drama by director Robert Zemeckis and starring Marion Cotillard as French agent Beauséjour and Brad Pitt as a Canadian spy. Their identities —and success — depend on how convincingly they cloak their true purpose. The same can be said of that silk satin dress, and much of the wardrobe created by costume designer Joanna Johnston, an Academy Award nominee for “Lincoln.” These are clothes meant to provide a mask for the performers and to usher the audience past history’s details and into a refreshed retelling.

Her vision of 1942 and 1943 was less an exercise in authenticity and more a straight-out seduction.

“I like to try and do things that look appealing to today’s eyes. I love it when an audience member says, ‘I just would love to wear that dress.’ You want people to have a crush on the clothing,” Johnston said by telephone from London. Toward that end, she channeled Hollywood’s greatest stars and designers and tailored the period costumes with modern eyes.

Though the gown recalls the slinky numbers that screen sirens like Rita Hayworth wore, Johnston designed it for action. Out went the square ’40s shoulders and in went the soft gathers.

“When she’s static, it has a good hang to the cloth. It’s sort of structured on the top, and more fluid from the waist down. I wanted her to look sort of goddess-y when she’s standing, and liquid when she’s in movement,” Johnston said. Like an action heroine, Cotillard wields a machine gun and sprints from an assassination in the gown.

With Pitt and Cotillard as the stars, Johnston envisioned a matinee-idol approach in a presentation she gave to Zemeckis. “I said, ‘Brad and Marion are so fabulous looking. Why don’t we give it this kind of gloss?’”

For inspiration, she absorbed the style of Barbara Stanwyck and Hedy Lamarr, as well as Humphrey Bogart and Lauren Bacall in 1944’s “To Have and Have Not,” and famed costume designer Orry-Kelly’s gowns for Bette Davis in “Now Voyager.” With the film opening in sultry Casablanca, Johnston felt obliged to reference the classic 1942 film of the same name as well.

“I looked at ‘Casablanca’ and was sort of amazed, particularly at Ingrid Bergman and how you could put most of her stuff on today and you’d look great,” she said. The same with Bogart’s trench coats, suits and tuxedos. “All of that is good and stylish now. And timeless.”

The menswear in “Allied” is of particular precision, giving Pitt a suave grace. He’s as swashbuckling as Rudolph Valentino in sweat-stained linens in the desert as he is in his precisely tailored uniforms and pale, monochromatic civilian suits and shoes.

“It’s very elegant for men, this period,” Johnston said. “I think Brad loved that tailoring. He’d been playing some gritty roles lately.”

Captivating as they are, few can upstage the spies’ infant daughter, Anna, in her knitted ensemble — a symbol of innocence in sugar pink.

And, much like the war years when all able-bodied persons joined in the effort, Johnston enlisted a friend’s mother to hand-knit multiples of the three-piece pram set in flecked Shetland wool. A strict production schedule meant a rigorous knitting schedule.

“She knitted day and night, and luckily, it was winter, so she couldn’t go galavanting outside because the days were short,” said Johnston. “But she was very brave about the hideous time schedules.”

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