Midway through Paul Thomas Anderson’s new film “Phantom Thread,” the main character — a 1950s fashion designer played by Daniel Day-Lewis — descends a staircase in his elegant London townhouse. He has worked a long day and taken a bath and is coming downstairs for an at-home dinner. He is wearing lavender pajamas that look to be made of insanely expensive cotton, with a cardigan sweater, a paisley ascot scarf and a tweed jacket. He looks comfortable, beautifully coordinated and utterly smashing.

And, according to the film’s costume designer, Mark Bridges, that particular styling was courtesy of Day-Lewis himself. The actor, known for immersing himself in his roles, put the outfit together from pieces in an extensive wardrobe provided to him — imagining what his character, Reynolds Woodcock, might wear on just such an occasion.

Bridges, in a phone interview last month, said he worked with Day-Lewis and Anderson to assemble several clothing pieces for the character — vintage and custom-made.

“We created his wardrobe, and we put it in his room as a closet,” Bridges said. “So he was able to walk in there and dress as he felt Reynolds would dress at any given time. He would text me sometimes, ‘Does that seem all right, are you OK with that?’ Without exception, I felt, if it was coming from a place of creation with him, then it would be fine for me.”

For the dinner scene, “Paul and I waited with bated breath to see what was going to come down those stairs!” Bridges said. “I was really thrilled with it.”

Bridges, an Academy Award-winning designer (for 2011’s “The Artist”; he was also nominated for Anderson’s 2014 “Inherent Vice”), has worked with Anderson various times before, but never on a film quite so costume-focused.

For “Phantom Thread,” he created not just period-appropriate wardrobes for Day-Lewis and the other characters, but an entire spring line for a House of Woodcock fashion show.

It was a fascinating time for fashion, coming shortly after the postwar birth of Christian Dior’s ultrafeminine New Look — which, Bridges explained, “was a reaction to the end of some of those wartime deprivations, the scarcity of things.” Clothing rationing in the U.K. had only just ended in 1949 (Queen Elizabeth II famously had to save ration coupons for her silk wedding dress, two years earlier) and couture customers happily dived into luxury again.

‘Method designing’

Real-life midcentury designers such as Balenciaga, Charles James and Dior inspired Anderson’s script, although the character of Woodcock is fictional. And that character gave Bridges an interesting challenge: For the Woodcock-designed pieces, the designs had to be guided not just by Bridges’ preferences and research, but by what Woodcock’s preferences might have been.

“I called it, jokingly but not jokingly, Method designing,” said Bridges, who found himself frequently consulting with Woodcock’s avatar, Day-Lewis. With Anderson, the three came up with a distinctive look for the House of Woodcock: “heavy, rich colors, heavy fabric, velvet, satin, heavy doses of lace. And with some historical references.”

During the design process, Bridges would “oftentimes go to Daniel and say, ‘Would you like to choose the fabric for this? Or a color?’ ” Bridges said.

For an antique-lace gown worn by Reynolds’ muse Alma (Vicky Krieps) in a key scene, Day-Lewis/Woodcock chose the color, a pale mauve. “I knew I wanted it satin, so I gave him the satin [sample] book and he chose the color. I thought it was beautiful together. If I’d had a problem with it, I could have said something. Both Paul and I wanted Daniel as Reynolds to have a feeling of authorship with some of these clothes.”

Before the film’s shoot, Day-Lewis spent months learning traditional dressmaking techniques, including a lengthy apprenticeship in the costume department at the New York City Ballet. He even painstakingly re-created a 1950s Balenciaga gown — using his patient wife, writer/filmmaker Rebecca Miller, as a fitting model.

“He got very proficient,” Bridges said. “It helped him really be believable, when he’s doing that draping and cutting, using his eye and taste to check proportions.”

Upon seeing the finished film recently, Bridges was struck by the realistic nuances in Day-Lewis’ performance. “I thought it was a great job, of him really looking and assessing,” he said, noting “details like him turning her to the mirror and then looking at [the gown] in the mirror. That’s something I do a lot, when I’m doing a fitting. I felt like he’s really well observed behavior in a fitting room, certainly to the point where it seemed really flawless to me.”

50 garments created for film

Bridges created about 50 garments for the film, all constructed from scratch in the traditional couture manner: first creating toiles (mock-ups, using plain muslin, to determine seam placement and fit), then final versions using the actual fabric. Like Day-Lewis’ preparation, it’s meticulous work, but it added to the film’s realism.

“The happy byproduct of that was that once they were cut into the real fabrics, the toiles could come on the set and be part of the set dressing,” Bridges said. “Many of [the costumes] were supposed to be House of Woodcock creations, so they were added to the atmosphere.”

Although period films generally draw more attention for their costumes, Bridges most frequently works on contemporary films, with recent projects including “Silver Linings Playbook,” “Jason Bourne” and “Fifty Shades of Grey.” (It was, he said, “great fun” costuming Christian Grey — “you don’t often get to go shopping for a billionaire character!”) He’ll next design a contemporary film, reuniting with “Greenberg” director Noah Baumbach.

But whether it’s couture gowns or modern-day jeans, the role of the costume designer remains the same, Bridges said: “to give quiet emphasis to the drama and to subtly tell that story.”

In “Phantom Thread,” he said, “yes — you notice the dresses. They’re part of the story. But I’m happy that they don’t upstage or take away from Alma and Reynolds; they’re there as a backdrop to emphasize and illustrate that world.”