ABC News' Victoria Thompson ( @VicThompsonABC) and Lauren Effron ( @leffron831) report:

"Mad Men," a show about the sin, swagger and back-stabbing salesmanship of a New York City advertising firm in the 1960s, not only became a smash hit TV show, but also launched a style revolution and a cultural phenomenon.

"It's a pretty cool thing when you think about it," actor Jon Hamm, who plays the silver-tongued Don Draper, ready to woo any woman with a single light of his cigarette, told ABC's Chris Connelly.

The clean-cut style of the "Mad Men" characters, which later spawned a successful Banana Republic clothing line, was originally the vision of show creator Matt Weiner, said actor John Slattery, who plays the no-nonsense ad boss, Roger Sterling.

"Matt was telling me early on and still does, that it needs to be more formal because I'm sort of more slouchy," Slattery said. "But the clothes go a long way. It helps because you really can't breathe or slouch, or do anything but stand up straight if you want to breathe at all."

Watch the full story on " Nightline" TONIGHT at 11:35 p.m. ET/PT

For the men on the hit AMC show, the straight-cut business suit and an ironed pocket square. For the women, dresses with a clinched waist, lots of floral patterns, multiple strands of pearls and heels that are no longer just your grandmother's pumps.

"Nightline" was granted exclusive access inside the show's costume closet with "Mad Men" costume designer Janie Bryant, whose team creates looks that are now synonymous with the show.

PHOTOS: "Mad Men": Behind the Scenes on Set

Actress January Jones, who plays Don Draper's icy ex-wife Betty, said the clothes help the cast develop the characters they play on-screen.

"We come [to the costume closet] before each episode to get fitted, and it just kind of starts the creative process, at least for me, to get to know what I'm doing, and what I'll be wearing," Jones said. "Through the course of the different seasons we've changed Betty's silhouette several times, and it's kind of a big part of the storytelling, whether she's been, you know, riding horses, or pregnant, or a politician's wife - or divorced."

Credit: ABC News/Victoria Thompson

The Drapers' divorce was a major turning point in the show's plot. Afterwards, Bryant said she moved Jones' character away from the cupcake dresses she wore as Don Draper's wife and into the pencil skirt-wearing politician's wife when Betty married Henry Francis in season four.

"The big influence for me, for Betty's character, then was Jackie Kennedy, as opposed to a big influence for me in season one, Grace Kelly," Bryant said.

Jones told Connelly she has nicknames for her outfits and one of her favorites was a dress she dubbed "sad clown."

"It was a polka-dot dress, and the episode started out very happy for Betty," Jones said. "I stayed in the outfit, and was progressively more miserable. The dress got more miserable."

As for the man himself, Hamm said he takes style cues from memories he has of his late father.

"I remember going in my dad's closet and seeing row upon row of suits, probably 70 suits," he said. "Who has 70 suits now? It's kind of a lost part of dressing up now, but I think the show's kind of bringing it back a little bit."

The Emmy award-winning "Mad Men" returns for its fifth season on AMC on March 25.