Deep in their under-actualized hearts, understudies are actors, too. : The role is the most demanding he has never played.

Bob Hackman has the most frustrating plum job in Hollywood.

Six nights a week Hackman does the professional equivalent of putting on a tux and not going to a party. Twice every Saturday and Sunday he has all the fun of a top-notch chef catering a fast.

Hackman, who lives in Canoga Park, is Peter Falk’s understudy in “Glengarry Glen Ross,” David Mamet’s hit play at the Henry Fonda Theater.

The job is a classic double-or-nothing gamble. Hackman, 57, a veteran character actor but hardly a household name, will star in the Pulitzer Prize-winning drama, playing the lead role of real-estate hustler Shelly Levene, if he ever gets to go on. When the call comes, if the call comes, Hackman will rise from the theatrical bench to play quarterback.

That image is particularly apt since what Hackman usually does during the play is watch football on a small black-and-white TV set in one of the backstage dressing rooms. He also studies his lines, since the role of Levene, who has monologues longer than Hamlet’s, with almost as much subtext and a lot more four-letter words, is the most demanding he has never played.

Actually, Hackman must be ready to assume either of two very different roles, the tough, desperate Levene, and a timid fellow hustler, played by Alan Manson. “The first time I rehearsed the play,” Hackman recalls, “we had to bring an exorcist in to take the one character out so I could do the other one.”

Hackman, who would be a St. Bernard if he were a dog, is at the theater 30 minutes before every performance, Mamet’s demanding dialogue unscrolling in his head. He knows the words, he knows the gestures, the inflections, the silences he would wield to bring the audience to their knees. He is ready. And Peter Falk hasn’t missed a performance yet.

It is a tribute to Hackman’s moral fiber that he doesn’t wish on Falk so much as a teeny-weeny fracture that would put him in traction for a month. “I don’t want to go on that way,” he says. Which is not the same as saying he doesn’t want to go on.

When Falk appeared recently on the Johnny Carson Show to talk about the show, Hackman thought he had finally got his chance. Falk wasn’t at the theater 10 minutes before curtain. At the five-minute call, Hackman had minimal powers of speech and had realized the true horror of the understudy’s lot. It’s bad enough that you may never get to perform but, when you do, it’s for an audience that has shelled out almost $30 a ticket to see someone else.

“They come to see Falk,” Hackman says. “What happens if I go on and Falk doesn’t go on? Are they going to come up onto the stage and drag me off and lynch me? That’s a concern.”

Well, yes, but deep in their under-actualized hearts understudies are actors, too. The fact that you rarely see them doesn’t mean that they’re shrinking violets. Self-effacing types don’t give up fringe benefits in perpetuity to pursue acting careers. So, while the stars star, the understudies sit in the bowels of the theater and swap tales of legendary backups who have gone out there and killed the initially disappointed multitudes.

How many minutes into “Picnic” did the audience still mind that Ralph Meeker had been replaced by his understudy, a guy named Paul Newman?

Hackman, who has done some strange things in the business, including playing a Mr. Whipple impersonator in a Charmin commercial, has never understudied before. He accepted this job, in large part, because it was too prestigious a role, however iffy, to turn down. The current company, headed by Falk and Joe Mantegna (who plays the womanizing periodontist in the movie “Compromising Positions” and has a way with more than dental floss), will go on to do a national tour. Hackman knows that his understudy gig could lead to an on-stage role in the troupe. Meanwhile, he says, “I’ve got a paycheck coming in for showing up an hour a night.”

Hackman remains at the ready until the stage manager assures him at the intermission that both Falk and Manson are prepared to finish their performances. Equity minimum for understudies is $750 a week. That’s less than Hackman makes when he works in films or TV but, as he notes, “Theater money, stage money, is never as good as movie money.”

But who cares? Doing Mamet goes a long way toward healing the damage done to an actor’s soul by working in one episode too many of “Craven & Craven” or “Celebrity Pets.” As Hackman points out, it’s only on stage that an actor gets to perform for living, breathing human beings. When sufficiently entertained, living, breathing human beings clap. In the best of all possible worlds, they clap standing up.

“The gratification is immediately transmitted to you,” Hackman says. “How much did Peter get doing ‘Colombo?’ $1 million an episode? I don’t think he’s getting $1 million a week here. He’s doing it because he loves it.”

The company regards its three understudies as valuable insurance and treats them accordingly, even graciously, Hackman says. On opening night, playwright Mamet gave him a kilted toy soldier with a good-luck message attached. All that is gratifying, but it doesn’t quite make up for the fact that Hackman hasn’t yet had a chance to act--the thing, by definition, that actors do.

“It’s not easy,” says Hackman, who adds, almost plaintively, “I’m an exhibitionist.”

The worst time was during the gala party thrown to celebrate opening night. “I felt like I wasn’t there, like I was The Shadow,” he recalls. The unforgettable affront was when the hired hand guarding the door eyed him suspiciously and asked if he had an invitation.

Hackman managed not to say, “You twit, I am the understudy to the star.”

But, then, Hackman always was a quick study.