“Miss Saigon” – isn’t that the megamusical with the helicopter onstage?

That’s not what director Eric Schaeffer and his cast of 19 are talking about at Arlington’s Signature Theatre as they sit in a circle two weeks before Thursday’s opening of the epic show — at least not now. Tasked with fitting one of the most famous blockbuster musicals ever into the intimacy of a 275-seat theater, they’re looking for the small human truth of Alain Boublil and Claude-Michel Schönberg’s 1989 follow-up to their world-conquering “Les Misérables.”

So Schaeffer has the cast doing “table work,” thinking about characters snared in a plot that grafts Puccini’s “Madama Butterfly” onto the 1975 fall of Saigon. Imagine: You’re in a sleaze-filled Bangkok, where U.S. soldiers and local prostitutes do business. What’s your attitude?

“Bangkok is the nastiest place in the world,” says Thom Sesma, who plays a small-time hustler known as the Engineer. “And none of us is here for anything else. Except for the refugees.”

“I saw some really disturbing things as a kid walking around the streets of Japan,” says Diana Huey. She’s playing Kim, the local girl smitten with the good-natured American soldier Chris, who has an American wife back home.

Signature Theatre presents “Miss Saigon” through Sept. 22. (Courtesy Signature Theatre/The Washington Post)

Sesma, who played the Engineer on the show’s second national tour from 1995 to 1997, later explains the impact of Signature’s close confines on a show that, at $10.9 million, cost significantly more 22 years ago than this nonprofit troupe’s entire eight-show slate will cost this season.

“The audience can see you sweat,” says Sesma, chatting in a small rehearsal room with Huey. “It changes everything.”

Even on its own limited terms, Signature’s “Saigon” in some ways will be the biggest venture yet for the company that likes to test its limits. An environmental set will loom all around the audience; raw material for Adam Koch’s design includes discarded components from real airplanes, including one said to have flown over Vietnam.

The troupe is trumpeting the show as “the most technically ambitious in the theater’s history,” and is selling $10 backstage tours under the heading “Dreamland: Saigon Backstage.”

The original production was a ballyhooed colossus by the time it made its way from London to New York in 1991. The show, which ushered in the $100 ticket, featured an 18-foot tall statue of Ho Chi Minh, an actual Cadillac for the Engineer to mount lewdly, 46 actors requiring 375 costumes, a crew of 55, plus that spectacular helicopter.

When it finally closed after almost a decade, the New York Times described “Miss Saigon” as “the most technically complex show in Broadway history.”

With 19 actors and an orchestra of 15, the scale is comparatively fractional at Signature, where the success of its own downsized, big-voiced and big-hearted 2008 staging of “Les Misérables” led to the next item in the Boublil-Schönberg catalogue. (To get the rights, Signature had to wrap up this production by October; the film success of “Les Miz” has spurred a new “Miss Saigon” production in London for next spring, with a Broadway transfer and a film version being discussed.)

Yet fitting “Miss Saigon” into the theater’s $7 million budget for 2013-14 required two sponsors underwriting about a 20 percent bump in resources just for this project. (Schaeffer declines to put a figure on the “Saigon” cost.) “Les Miz” demanded more staffing, but “there are more moving parts to this,” Schaeffer says.

Saigon and Bangkok are “big worlds,” Schaeffer says — and though, like a barker coyly drumming up intrigue outside the circus tent, he won’t say exactly how it’s going to happen, he has to deal with the helicopter.

“It’s in the music,” Schaeffer says during a break in rehearsal. “You have to honor that.”

He flips through an elaborate storyboard of sketches by Koch, who designed “Dreamgirls” and “Kiss of the Spider-Woman” for Signature and, in Memphis, another “Miss Saigon” earlier this year. He plays with the model, a charred-looking miniature of the complicated set with features that include a movable central platform.

Schaeffer and Koch started visualizing the show’s look last September. In January, Schaeffer had a revelation.

He was putting together a Las Vegas company of “Million Dollar Quartet,” the Johnny Cash-Elvis Presley-Jerry Lee Lewis-Carl Perkins catalogue musical he has directed through a long pre- and post-Broadway life, when he visited the Neon Boneyard, a museum of vintage signs from the casino strip’s yesteryear.

The Boneyard inspired a search for wartime relics. That led technical director Andrew Fox to a Craigslist hunt, which ended when a collector in Michigan sold Signature a truckload of used airplane parts.

Some of the parts sit outside on Signature’s loading dock: a fuselage, a couple of banged-up plane wings. Already onstage a week before the actors will arrive for technical rehearsals are tall metal radio towers. Two of them rise above the stage’s front corners by the audience, the better to envelop the patrons (who will sit on three sides of the stage) in the chaos of wartime. The towers were built two months ago and left outside to rust so they would look genuinely aged.

Also onstage: an old cockpit, seemingly abandoned, and an airplane wing that is a key part of what technical director Andrew Fox calls Koch’s “sculptural” set. The wing, about 15 feet long, is hinged to rise and fall. It’s a ramp for actor entrances and exits.

As Fox stands in the middle of the half-completed set, a crew member in a Genie lift drills a corrugated plastic sheet to metal framework over the stage. The company has installed a brand-new tracking system to handle three layers of panels that will slide in and out, illuminated by lights and projections as the story’s locales shift.

Back in the rehearsal room upstairs, the introspective table work is over. The actors are playing a sequence that shifts from seedy Bangkok to the mayhem in Saigon. Choreographer Karma Camp explains to the actresses that some of the prostitutes will be inside the metal towers — “Like they’re in go-go cages,” she says.

The cast performs without stopping for 10 minutes. When Schaeffer halts the action, his direction now isn’t “What is your character feeling?” It’s physical engineering, describing how things will work once they get on the busy set.

To one actor, he explains where his position as a soldier will be: up a tower working a searchlight. When the refugees ask how the American embassy’s fence will be rendered onstage, he says, “It’s an iron frame. Hit the gate there.” He points to a taped mark on the floor. “But then get back.”

Erin Driscoll, who plays Chris’s American wife in the show, has a new song that is getting its American premiere here. It’s called “Maybe,” replacing “Now That I’ve Seen Her,” and new lines may still be coming from the show’s co-lyricist, Richard Maltby Jr.

“This is so much better,” Driscoll says during a break.

A new song is significant, but it feels like a footnote, as does the fact that Schaeffer avoided the notorious casting controversy that dogged the show’s U.S. opening, when Englishman Jonathan Pryce was imported to play the Eurasian Engineer despite howls of protests from Asian American performers who felt they were being passed over.

Sensitivities were heightened last year during a heated dispute at San Diego’s LaJolla Playhouse. A workshop of “The Nightingale,” a new musical by “Spring Awakening” writers Duncan Sheik and Steven Sater, had a largely Caucasian cast that drew raised eyebrows, especially because a Chinese emperor was played by a white actor.

“Eric went to great lengths to cast this show authentically,” says Sesma, who is half Japanese. He adds that it’s certainly easier now than it was two decades ago: “Because of ‘Miss Saigon,’ an entire generation of Asian American performers blossomed.”

Huey, a non-Equity performer from Seattle (and born in Japan) who calls Kim “my dream role,” is proof. It turns out that the last role cast was Chris, as Schaeffer held out for the right combination of strength and vulnerability.

But the larger story at the modest-sized theater is size: more, most, bigger. In July, “Miss Saigon” registered Signature’s single biggest box office day ever in terms of dollars.

“People want to see this show,” Schaeffer says.

He guesses at the questions driving audience interest: “How are they going to do it? And is there a helicopter?”

Miss Saigon

music by Claude-Michel Schönberg, lyrics by Richard Maltby Jr. and Alain Boublil, adapted from the original French lyrics by Alain Boulbil. Directed by Eric Schaeffer. Thursday through Sept. 22 at Signature Theatre, 4200 Campbell Ave., Arlington, Va. Call 703-820-9771 or visit www.signature-theatre.org.