By Jill Serjeant

LOS ANGELES (Reuters) - Pick a social cause and you'll often find a Hollywood celebrity speaking out. Gay marriage (Brad Pitt), Darfur (George Clooney), the environment (Robert Redford or Leonardo DiCaprio).

Gun control? Not so much.

Most of Hollywood's biggest action movie stars have remained silent, so far, on the divisive issue following last week's slaying of 20 young children and six adults at a Connecticut school. And pop culture experts say it's not hard to see why.

"If you are known for being a star who carries around weaponry and fires it, when something like this happens, the last thing in the world you want to do is insert yourself ... unless you say you are never going to star in another action adventure movie," Robert Thompson, professor of popular culture at Syracuse University, said on Monday.

Longtime gun control advocates like actress Susan Sarandon and "Bowling for Columbine" documentary director Michael Moore were quick to take to Twitter after Friday's Connecticut massacre, and tens of thousands of Americans have since signed online petitions urging approval of stricter gun control laws.

Yet major action heroes Arnold Schwarzenegger, Bruce Willis, Denzel Washington, as well as Pitt and Clooney, have had little or nothing public to say.

That may change, according to a veteran public relations executive who handles many Hollywood clients.

"I think there will be a very public display of outrage from prominent people in the entertainment world and people wanting to do something about guns," said the public relations chief who asked not to be named because he was not authorized to speak for his clients.

"I think the stereotypical Hollywood action star not wanting to touch an issue like this may be dated. There is a lot of talk about people wanting to express very public outrage. Let's see who joins, and how that manifests itself," he told Reuters.

CRUISE IN SPOTLIGHT FOR "JACK REACHER"

With Hollywood studios again under scrutiny for making violent movies, Paramount Pictures canceled Saturday's premiere in Pittsburgh of Tom Cruise's new film "Jack Reacher," in which Cruise plays a cold-blooded former military sniper.

In New York, the Lincoln Center Film Society postponed a Monday screening and conversation with Cruise "out of respect for the families who lost loved ones in Newtown, Connecticut," according to a statement.

"Jack Reacher," which opens with a sniper picking off and killing five apparently random targets on a riverfront promenade, is due to open in U.S. movie theaters on Friday.

Cruise has said nothing publicly about the events in Connecticut.

But "Jack Reacher" director Christopher McQuarrie told entertainment industry website TheWrap.com that the actor played a key role in the decision to cancel the red carpet premiere in Pittsburgh - where much of the movie was shot.

"Tom and I insisted on it. Nobody should be celebrating anything 24 hours after a tragic event like that," McQuarrie told TheWrap on Monday. Continued...