PUNCHLINE * * * 1/2 Sally Field, Tom Hanks, John Goodman, Mark Rydell; rated R (profanity, vulgarity); Century Complex, Cineplex Odeon Regency Theater, Cineplex Odeon Trolley Corners Theaters, Mann 6 Plaza Theaters.

About halfway through "Punchline" Steven Gold is standing before a microphone in The Gas Station, a New York comedy club where he's been performing nights for 18 months, flunking out of medical school along the way.Like most standup comics, Gold finds his comedy from painful real-life experiences, in his case purging guilt and misery over being forced to work toward a medical career by his domineering doctor-father.

On this night he's especially wound up and on edge. He's been told someone out front has been asking about him, and he's sure it's a network talent scout he's been expecting.

Just before he jumps onstage, however, Gold glimpses the table where this important visitor is sitting, and from the look that crosses his face we know immediately it's his father.

Gold's finely tuned comedy routines vanish from his mind and he takes the microphone only to stumble over the worst case of flop sweat in show business history. It's his nightmare come true. And it's a turning point in his life.

Up to now he has now been self-centered and obnoxious to everyone he meets, but now he changes. He doesn't simply see the error of his ways and become Mr. Nice Guy, however. Instead the experience crystalizes his arrogance. Now comedy really is everything.

"Punchline" is a very funny movie, but it's also a very sad movie, with something to say about how obsession drives artists - in whatever art form - to become the best. If they don't have the talent anyway, forget it. But even if they do, the obsession is what ultimately puts them on top.

Gold is obsessed.

On the other hand, Lilah Krytsick, a New Jersey housewife, is not really obsessed. She's also not very good. Does she have what it takes to do standup comedy? At first we're not sure, but Gold takes her under his wing and teaches her how to be better. And how to become obsessed.

Along the way he thinks he's falling in love with her. But despite her bad jokes about her Polish husband and whiny kids, Lilah loves her family. She's not about to give that up for the obsession. And she's smart enough to see that Gold is desperate and needs somebody - anybody.

But the actors aren't just anybody - some wonderful real-life comics spot the supporting cast; John Goodman stands out as Lilah's husband; director Mark Rydell is excellent as the owner of the nightclub; and Sally Field is quite good - and convincing - as Lilah.

But the picture belongs to Tom Hanks, in a very risky role as the none-too-likable, but effortlessly talented and charming Steven. This is, I know, an overused superlative - but Hanks is brilliant.

Most powerful for me was the scene when Steven's emotional instability becomes most obvious as he does a "Singin' in the Rain" dance on a dark, wet street as Field watches from a diner and realizes he's losing it. It's a most moving moment. Mr. Hanks, you're Oscar nomination is in the bag. Maybe your Oscar, too.

On the whole, "Punchline" is a very good film by David Seltzer, who wrote (as he did "The Omen") and directed (as he did "Lucas"). There are some lapses in credibility that do temporary damage to the narrative structure, but nothing fatal: Lilah's husband, who, through most of the film, is completely against her pursuing a comedy career, turns around too quickly and completely; there are comic scenes that, though funny, seem out of sync with the film's tone; and the finale is too pat and sentimental.

As Gold says at one point, "There's nothing funny about comedy." And if "Punchline" is any indication, that's an absolute fact. There's no subtlety to the message that comedy is hard and that comics are pained - and sometimes just pains.

As you might expect, the R rating is for language, and there are some monologues that are more raunchy than funny - but that's certainly in keeping with with modern comedy.