IT CAN BE SAID, with little fear of contradiction, that Scott Greer is currently performing the biggest role of his career.

Last night, he opened in Theatre Exile's production of "The Whale." In Samuel D. Hunter's drama, Greer, a longtime pillar of the regional-theater scene, portrays a 600-pound man named Charlie, struggling to reconcile with his daughter as he grows progressively ill.

To convey Charlie's morbid obesity, costume designer Alison Roberts and her team conjured a costume weighing in at around 50 pounds.

"I've never [worn] anything like it," Greer said. "I've done shows where I used puppets and stuff. This is like operating a puppet from the inside. That presents a challenge. It's very heavy and cumbersome. After a couple hours, you feel it."

Actually, in terms of the physicality of the role, Greer admitted that he doesn't have it all that tough, given that much of his time onstage is spent either in a wheelchair or on a couch. The real challenge, he has found, is dealing with the near-constant wheezing that Charlie endures because of his constrictive heart failure.

He described having to "figure out how to do that without hyperventilating and passing out - which actually happened the first time I did a reading of the play. I passed out at the table.

"The demands are unique," he concluded.

Greer, a 2001 Barrymore Award winner for Outstanding Actor In a Play for InterAct Theatre Company's production of "It's All True," is exuberant in his praise for "The Whale."

"The play is just really, really . . . beautiful," he said. "[Charlie] is this unbelievably sweet guy and sort of relentlessly positive despite his circumstances.

"I love the tension the playwright has created. All of the characters are very hard to [like]. You have compelling reasons to be repelled by them and, at the same time, compelling reasons to be drawn to them, [Charlie] most of all. It's sort of tough to watch somebody like him."

Although he has plenty of dramatic roles on his resume, Greer is probably better-known for his comedic work, especially with 1812 Productions, the nation's only comedy-exclusive theater company, co-founded by his wife, Jen Childs. His next project is "To The Moon," a comedy inspired by "The Honeymooners" and penned by Childs.

Like so many others before him, he confirmed that doing comedy is tougher than performing drama.

"If a line is designed to get a laugh and it doesn't, that's hard," he explained. "Drama doesn't ask you to engage in the same way. An audience's expectation is different, and a performer's expectation is different.

"People watch dramatic plays in a different way. No one ever leaves a drama disappointed that they didn't cry, whereas if you leave a comedy and didn't laugh once, that's tragic."

Comedy "requires not only truth, but precision. In most drama, truth is very important, but precision less so. It's a very delicate thing. Somebody can cough in the wrong place and half the audience misses a punch line."

Theatre Exile, 1340 S. 13th St., through March 1, show times vary, $10-$50, 215-218-4022, theatreexile.org.

Dated 'Private Lives' at Walnut Street

It's easy to imagine the impact Noel Coward's comedy of manners "Private Lives" had on audiences when it debuted in 1930. Back then, subjects like divorce, infidelity and promiscuity were generally considered taboo in polite society.

But given that most popular entertainment these days is predicated on those subjects, the Walnut Street Theatre's production, while entertaining enough, seems more like a museum piece.

The mustiness starts with the setup: Amanda and Elyot - two elite Brits - have divorced and remarried and find themselves honeymooning in adjoining rooms in a luxury French hotel. Virtually the minute they lay eyes on each other, passions rekindle and the two split for Paris, leaving their respective new spouses in the lurch.

It's all very sophisticated and witty, but it's also talky, and the third act is as much a dead weight as a denouement.

The four principals of the five-person cast - Kathleen Wallace, as Amanda; Greg Wood (Elyot); Dan Hodge (Victor); and Lauren Sowas (Sibyl) - are all spot-on, to use a phrase their characters might have employed. But the show belongs to Wallace, whose Amanda keeps hopscotching between Katharine Hepburn and Lucille Ball, sometimes in the same paragraph.

It's a somewhat bizarre performance; at times, it's downright schizophrenic. But it's also a memorable one that injects a much-needed buoyancy into "Private Lives."

Kudos also to scenic designer Robert Koharchik, whose two sets are eye-popping (especially Act Two's rendering of an Art Deco Paris apartment).

Walnut Street Theatre, 825 Walnut St., through March 1, show times vary, $20-$85, 215-574-3550, walnutstreettheatre.org.