Rooney Mara in “The Girl with the Dragon Tattoo.” (Columbia TriStar Marketing Group, Inc.)

The new science fiction ad­ven­ture ­­“Looper” bears the unmistakable imaginative signature of its maker, writer-director Rian Johnson. But it arguably exists because of one man: Bruce Willis.

“Looper,” a smart, stylish drama that combines serious themes with slick effects, features many accomplished actors, among them Joseph Gordon-Levitt, Emily Blunt and Jeff Daniels. But it’s Willis’s involvement that made the modestly budgeted independent film possible. “The instant that you have Bruce Willis attached to a science fiction movie you’re off to the races,” Johnson said this month at the Toronto International Film Festival, where “Looper” was the opening-night film. “The reason the movie got made was because Bruce signed on.”

In an artistically pure world, casting a film means finding the right actor for the right part — a convergence of means and ends (not to mention salaries and budgets) that results in the Platonic ideal of the filmmaker’s vision. More than ever, though, casting has become a crucial part of the business side of movies, from courting financial backers and distributors to gleaning free publicity in an increasingly expensive marketing environment. As Johnson himself admits, the quality of Willis’s performance was the last thing on the minds of investors whom the film’s producers approached to underwrite its $60 million budget.

“There is that unfortunate side to it, when you’re dealing with this business model, where there are horrible, horrible lists that no actor should ever lay eyes on, which lists their name and their foreign value,” Johnson said. “They reduce these actors to a number . . . based on what their previous movies have made box office-wise. So there will be these huge names that are just legends and extraordinary actors, and you’ll say, ‘What about this guy?’ and they’ll say, ‘No, he has no value.’ ”

These rules of the road became controversially clear this summer when it was announced that Zoe Saldana would play Nina Simone in a coming biopic about the late jazz singer. The choice created a firestorm among bloggers and commenters, who were aghast at the idea of Saldana playing a woman to whom she bears little physical resemblance. Shrewd observers of the industry, though, recognized that Saldana is one of a few actresses of color with the box-office pull — real or perceived — to get an independent production off the ground.

‘Substantial investment’

When big Hollywood studios prepare to adapt comic books or teen-novel franchises, they’ll often go with unknown actors in the roles, reserving their big-bucks spend for special effects and marketing and hoping that finding the next Daniel Radcliffe or Kristen Stewart will provide a publicity bump in itself.

But for filmmakers who work independently, with a fraction of a blockbuster’s budget, a big-name star has become more crucial as the industry has globalized. Just as guns, explosions and special effects are understandable in any language, Angelina Jolie, Johnny Depp and Richard Gere are just as recognizable in Beijing as in Biloxi.

“At this point in independent cinema, more than ever the financing is driven by the territories outside the United States,” says Nicholas Jarecki, who cast Gere in his Wall Street thriller “Arbitrage” after Al Pacino dropped out. Jarecki estimates that half a film’s budget is financed by foreign distributors. “They’re making a substantial investment, naturally they want to protect their investment, so they want a marquee cast that can attract filmgoers to the theater.”

Fred Roos, a producer who started out as a casting director on such films as “Five Easy Pieces” to “The Godfather,” says he finds the redoubled emphasis on casting “kind of a frustrating new aspect” of the film business.

“Say you somehow got the money together and made a really good independent film by a new director who has never done the festival circuit and you have nobody in it,” he says. “You have an almost impossible task of getting it picked up.” (Exceptions to the rule slip through the net occasionally, he notes, such as this summer’s art house favorite “Beasts of the Southern Wild,” which featured the unknown child actress Quvenzhane Wallis in its breakthrough starring role.)

Roos adds that the thrill of discovery has also diminished in a system geared toward big names and sure things. “For a producer — particularly a producer like me, who prides himself in spotting talent early — that advantage is not there [anymore]. Because the [financiers] are always a year or two behind on who they think is a name. So you can have somebody you know in six months is going to break big, and they’ll say, ‘I’ve never heard of him.’ ”

‘They mean something’

When producers of the independent film “The Butler” announced in March that Forest Whitaker would star in the Lee Daniels adaptation of Wil Haygood’s Washington Post story about longtime White House butler Eugene Allen, no doubt some potential investors took note of the Oscar winner’s involvement. Then, over the past several months, actors such as Robin Williams, Jane Fonda, Mariah Carey and Oprah Winfrey — to name just a few — signed on in supporting roles, drumming up early public awareness of the film but also sending important signals to would-be financiers and distributors.

Billy Hopkins, the veteran casting director who worked on “The Butler,” says the all-star lineup of the movie is a function of the new realities of financing, especially international funding. “We ended up with a really fine, wonderful actor playing [the title] part,” he says. “Did he make all the financing fall into place? Not necessarily. So you have to try to surround him with actors who do mean something financially and are also good actors.” (“The Butler” was recently acquired by the Weinstein Co., suggesting that it will fashion an Oscar campaign around the film next year.)

“I think we were fairly successful in terms of the actors we cast and also in terms of some of the other butlers,” Hopkins continues. “So it wasn’t just one-sided. So we got Cuba Gooding Jr., Terrence Howard, Lenny Kravitz and Oprah Winfrey. Then you have names like John Cusack and Robin Williams and James Marsden in the presidential roles. And they all mean something [to international investors] to varying degrees — I don’t have those lists, but add them all together and they mean something. Then you have my best idea, which was casting Jane Fonda as Nancy Reagan.”

Hopkins adds that it was no coincidence that much of “The Butler’s” casting news broke during the Cannes Film Festival, where the film was selling briskly to international distributors at the market — almost certainly because of names such as Matthew McConaughey and Nicole Kidman popping up in the press as possible cast members. (Both later dropped out of the production.)

“I often get hired to cast lead roles to coincide with Cannes or those other markets,” Hopkins says, adding that he’s seen the lists to which Johnson referred to, recording actors’ names and their box office value. “Sometimes it really is depressing. And the things they can say about actresses are just downright cruel.”

Scarlett’s legacy

Casting drama has always been the subject of public fascination — as far back as 1936, when producer David O. Selznick masterminded a national casting call for Scarlett O’Hara in “Gone With the Wind.” Nearly every actress in Hollywood — from Jean Arthur to Shelley Winters — either auditioned or screen-tested for the role until Vivien Leigh nailed the part. More recently, every hot Hollywood ingenue appeared determined to go out for the American version of “The Girl With the Dragon Tattoo,” for which David Fincher interviewed seemingly every actress of a certain young age, from Carey Mulligan to Emma Watson. (Rooney Mara finally landed the role, which earned her an Oscar nomination.)

Tony Gilroy’s casting process for “The Bourne Legacy” played out in a similar public way, not through any design of his. “Maybe it was good for them in a ‘Gone With the Wind’ kind of way,” he said of Fincher’s much-hyped search for the new Lisbeth Salander. “For us, it did not help. We came out of it with the same perception we had when we came in, which was that we wanted to replace Matt [Damon]. So it was bad for us.”

Still, if the “Bourne Legacy” casting process didn’t help Gilroy with message control, he said he did come to value it creatively. “It’s a lot of hard work and a lot of travel and time sitting around and b-------ing with people,” he noted, adding that he would often meet with an actor to ensure his agent’s cooperation on future projects.

“But I got to meet all of the timber for everything that I could conceivably build in the next 20 years. It was very illuminating. I found people I had been excited about to be completely fraudulent and others who are so cool you just want to be in business with them.”

By the way, Jeremy Renner finally landed that “Bourne Legacy” role, and a sequel is reportedly in the works. Meanwhile, the carousel of casting, creativity, money and marketing continues its symbiotic spin: This week, it was announced that Jake Gyllenhaal was in talks to star opposite Hugh Jackman in a crime drama called “Prisoners” — barely a day after his new movie, “End of Watch,” tied for first place at the box office.

“In some ways, it’s the way the hot dogs are made,” Johnson said ruefully. “In another way, though, it’s allowing these mid-range movies to be made in this independent way, which I think is really interesting.”