One “Boardwalk Empire” scene shows an older man on a sofa seducing a younger woman in a dress.

OK, there are many such scenes in the first season of “Boardwalk Empire.” But while filming the scene in question, the girl’s outfit kept falling off. And not necessarily because the script called for it to do so.

It disintegrated. Between each take, wardrobe staff hurriedly worked to repair the garment, an example of what happens when you use vintage clothing at least 90 years old. By the end of the scene, all that remained were patches of shredded silk.

Welcome to the world of period drama.

Viewers drawn to HBO’s new series will no doubt be dazzled by a set design that transports them to 1920s Atlantic City. But all the period detail in the world would mean nothing if those strolling the boards wore chinos and Doc Martens.

“We had to costume more than 6,000 people, and, of course, nobody can show up with anything at home that would look like Atlantic City in 1920,” costume designer John Dunn said. “We basically had to develop a department store.”

In size and scope, the only project Dunn has taken part in that compares to outfitting “Boardwalk” was the Bob Dylan movie “I’m Not There,” which bounced between periods of Dylan’s life.

Beyond scouring the country for period garb and fashioning what costumers could not find, the tailor and costume shop kept busy mending vintage clothing throughout filming of the first 12 episodes.

“Some of my shirts just fell off because it’s so delicate,” said Anthony Laciura, who plays Eddie Kessler, Nucky Thompson’s assistant. “That’s how accurate (it is). … Hats, overcoats, collars, all of these things. It’s so believable.”

The process began in April 2009, in preparation of shooting the series pilot two months later. Dunn’s team did meticulous research, examining old photos, studying tailoring of the time and using old books to pinpoint the exact silhouettes of the suits worn by characters.

While the production relied heavily on vintage clothing — “We were like detectives, combing through the country for resources for period clothing,” Dunn said — it also required custom-made clothing for all of the principal actors.

Down to the skivvies, by the way. Dunn, who’s worked on period films such as “Casino” and “Factory Girl,” finds actors can start to find their character if wearing period-specific clothing from head to toe.

“When you put on those clothes and you look around and you see 50 extras wearing the same clothes ... you land in that era,” said Shea Whigham, who plays Sheriff Elias Johnson on the series.

It helps that every stitch of clothing was made from fabrics in use in 1920. Whigham can attest to that. There is, he said, a weight to his “Empire” wardrobe that sets it apart from anything else he wears.

“The layperson may not notice, but the weight of a suit and how it fits a person is different from a contemporary Armani suit made of soft woolen,” Dunn said.

For all the investigation into stitching and fabric, there is one other aspect to costuming “Boardwalk Empire” that vexed Dunn. Look at photos of the time period. What do you notice?

That’s right. They’re all in black and white.

“Boardwalk Empire” is not. So, like production designer Bob Shaw trying to figure out what color palette to use when building interior and exterior sets, Dunn was tasked with discovering how vibrant wardrobes of the time were.

“It was just very interesting to discover there was a great deal of color,” he said, pointing to the use of purples, mustards and baby blues in the series. “I had to negotiate how much color to use because it would look strange to the contemporary eye.”

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