What’s the hold up on Men in Black III? Will Smith doesn’t like the script.

Shooting on the big threequel has been delayed because its lead star wants to see some changes, according to the new issue of The Hollywood Reporter (now on newsstands).

Though the script’s original draft (penned by Tropic Thunder writer Etan Cohen) found favor with the studio, Sony Pictures, director Barry Sonnenfeld and producer Walter Parkes, Smith wasn't wild. "He's become very enamored with aspects of screenwriting," says a source involved with the production.

Nevertheless, in an unprecedented move, Sony started shooting in November — with only one act of the script completed.

PHOTOS: New Generation of Superheroes

Sony spokesman Steve Elzer tells THR the studio came up with the unusual shooting plan because it feared the New York incentive program would expire at the end of December. The studio also has said the hiatus would allow outdoor scenes to be shot in New York in spring.

But several observers suspect the studio moved ahead with production largely because all of the key players -- including Smith, Tommy Lee Jones and director Barry Sonnenfeld -- were finally ready to go, and a delay might have jeopardized that.

It had built in a break in production that was scheduled to last from late December through mid-February, during which the remainder of the script was to be finished.

The problem still hasn’t been resolved – and now the hiatus has been extended until March 28 as a new writer David Koepp (who did uncredited work on the first MIB) is brought in for revisions.

PHOTOS: THR Party Pics

Koepp has hit work cut out for him.

MIB III calls for Smith's character, Agent J, to go back in time - 1969, to be exact – where he encounters famous figures of the day, like Yoko Ono, as well as a younger version of Jones' Agent K (played by Josh Brolin).

"It's hard because you're locked into the beginning of the movie," a production source acknowledges. PHOTOS: Hollywood About Town

One former studio chief is not surprised that Sony did not come up with a script that passed muster with Smith in the time allotted. According to a source with firsthand knowledge of the situation, Sony expected to save more than $35 million thanks to the New York tax program. (MIB has a budget that will easily pass $200 million.)

PHOTOS: Hollywood’s New Young Guns

Sony maintains that the extra costs are not substantial. "Because we extended the hiatus from the holidays, few people were on the payroll, so this was a relatively inexpensive decision that has had an insignificant impact on the budget," Elzer told THR in an e-mail.