LOS ANGELES — There’s a reason Hollywood calls it “development hell.” Even when it turns out well.

Such is the lesson of “Knight and Day,” a big-budget action comedy starringTom Cruise and Cameron Diaz that’s set to open on June 25, directly opposite Adam Sandler in his own comic romp “Grown Ups.”

As it happens, Mr. Sandler passed up a chance to star in “Knight and Day” about five years ago, when the project was known as “Wichita.”

That was before it morphed into “Trouble Man,” a quasi-romantic vehicle for Chris Tuckerand Eva Mendes, but well after it was dreamed up, by the writer Patrick O’Neill, as “All New Enemies,” a sophisticated R-rated caper in which an off-kilter older guy who behaves a bit like Peter Falk in “The In-Laws” was paired with a troubled young one, like, say, Edward Furlong of “Terminator 2: Judgment Day.”

The coming spring-summer blockbuster season is chock full of sequels like “Iron Man 2,”from Marvel and Paramount, and adaptations like the graphic-novel-based “Scott Pilgrim vs. the World,” from Universal.

But “Knight and Day” was made the old-fashioned way: by running an original, “spec” script through Hollywood’s brutal development mill, with its endless rewrites and changing star and filmmaker alignments. Finally, against all odds, it popped up as an event film on the summer schedule at 20th Century Fox, in partnership with New Regency Pictures.

“Knight and Day” is also notable because with it Mr. Cruise, once the most bankable star in Hollywood, is looking for his first runaway hit since “War of the Worlds,” which took in $234 million at the box-office in 2005.

The director of “Knight and Day,” James Mangold, is still finishing the film. But its trailershave turned into hot commodities on the Internet.

EW.com’s PopWatch column was among the first to voice approval, when its writer said in December: “I still don’t know exactly what it’s about, and that’s what’s so cool. What’s even cooler? These two superstars reminding us why they’re superstars.”

Actually, the story finds Ms. Diaz, as June Havens, trying to figure out what is real and what is not — not unlike Audrey Hepburn opposite Cary Grant in 1963 in “Charade” — after Mr. Cruise’s maddeningly unreliable secret operative, Roy Miller, takes her on a tear. Possibly in the service of a world-saving mission. Or not.

Most scripts die in development. Somehow “Knight and Day” survived. Asked why, Steve Pink, a producer of the film who helped work out the original idea with Mr. O’Neill, a high school friend from Evanston, Ill., pointed to its core concept, a shifty protagonist. “That, and the strength of Pat’s writing,” he said last week. “Otherwise I have no idea how it stayed alive.”

Mr. Mangold, Mr. Cruise and Fox executives declined to be interviewed about the film, which trades on plot twists, misdirection and big reveals. But several people who worked on the movie — most of whom spoke on condition of anonymity to avoid conflict with studio executives and others — described it as having gone through twists of its own, without quite as many laughs.

Mr. Sandler was offered a shot at the project about the time it was first set up, as “Wichita,” at Revolution Studios, with Mr. Pink and another producer, Todd Garner. It was Mr. Garner who had suggested that the troubled young counterpart to Miller, or Milner, as he was known then, become a woman.

Still, Mr. Sandler passed, declaring, by one account, “I just don’t see me with a gun.”

Revolution then folded. But its chief executive, Joe Roth, joined Mr. Pink and Mr. Garner in moving “Wichita” to Sony Pictures, by now with Chris Tucker, the madcap star of the“Rush Hour” series, cast opposite Ms. Mendes, who was riding high after appearances in“2 Fast 2 Furious” and “Hitch.”

Mr. O’Neill was still the writer. And Phil Joanou was supposed to direct — until Mr. Joanou’s “Gridiron Gang” opened poorly, and Sony got cold feet about the whole thing.

From there the project was off to Fox, with a new director, Tom Dey, who had just done the romantic comedy “Failure to Launch.”

Somewhere along the line “Wichita” had become “Trouble Man.” But Fox executives wanted more romance, so they brought on new writers, including Dana Fox, whose credits include “What Happens in Vegas.”

Eventually, according to the Studio System, an industry database company that is owned by The New York Times Company, more than a half-dozen writers followed Mr. O’Neill on “Knight and Day.”

Mr. Tucker and Ms. Mendes, however, dropped out, as did Mr. Dey.

Then Ms. Diaz, who had starred in “What Happens in Vegas,” stepped in. So did Gerard Butler. Almost. On meeting with producers, however, Mr. Butler announced that just that day he had agreed to do “The Bounty Hunter,” a competing comic caper, which paired him with Jennifer Aniston.

This left the door open for Mr. Cruise, who was on the rebound from a stretch of bad publicity and, on the strength of his widely praised performance in “Valkyrie,” was seriously auditioning at least five of Hollywood’s hottest projects as his prospective next film.

One of the five became “Salt,” a thriller set for release by Sony on July 23, with Angelina Jolie in the role that had been set for Mr. Cruise. Another was “The Tourist,” which now pairs Ms. Jolie and Johnny Depp in a thriller that has been shooting in Venice, for eventual release by Sony.

But Mr. Cruise became fixed on “Knight and Day.” Or, at least, on his vision for it.

An inveterate tinkerer, Mr. Cruise typically plays with ideas, characters and scripts, developing his slant on stories that are ultimately written by others. In this case, according to one person who was briefed on his work, Mr. Cruise wanted to superimpose a character of his own on top of Milner. Ms. Diaz too had ideas.

All of those became grist for Mr. Mangold, a writer-director with whom Mr. Cruise had nearly made “3:10 to Yuma,” before leaving the lead roles in that one to Russell Crowe andChristian Bale.

One Cruise touch in “Knight and Day”: a trailer moment, in which Ms. Diaz’s character flips over the handlebars of a motorcycle, fully armed, facing Mr. Cruise as he steers.

As of last week the Writers Guild of America West had yet to determine final writing credits for the film. But Fox, in submitting the project for credits arbitration, said it viewed the story as having been written by Mr. O’Neill, with a script by Mr. O’Neill and Mr. Mangold — a tribute to the staying power of the original story, notwithstanding the many writers who were involved.

Mr. Pink, for his part, is pleased that Hollywood, at least occasionally, is still willing to go through the bother of developing an original script, however grueling the process may be.

“You have an idea,” he said, “and it’s a miracle it ever happens.”