Boy, the cast of Broadway’s “On The 20th Century” is a wreck. Peter Gallagher, for one, was out of the show for a couple of weeks with a sinus infection that got really bad. Kristin Chenoweth has been performing with a cracked rib.

So why does everyone in the Roundabout Theater Company revival seem so happy?

All the actors seem to relish bringing back what they see as a gem of a show, the 1978 musical by Cy Coleman, Betty Comden and Adolph Green. Plus, the musical has a slapstick brand of comedy that everybody says they have great fun playing. Mark Linn-Baker gets both a spit-take and a glass of water in the face.

Andy Karl, the “Rocky” actor who plays a vain 1930s movie star, has a grown a truly smarmy mustache that’s very funny onstage, but less so in real life. “Oh, it’s awful,” Karl said after the show’s March 12 opening. “I went out to get a salad for lunch and the 14-year-old girl who took my order just laughed at me. She couldn’t take her eyes off it.”

Gallagher hated to miss those shows, but he had to get better. How’s he feeling now? “With my fingers, as my father would say,” he cracked.

For Chenoweth, the show gives her a chance to show off both her operatic training and her comedy chops. “It’s the role of a lifetime for a comedienne who’s an operatic singer,” she said. “Those don’t come around very often!”

After Gallagher’s absence, the opening night and the party went on as planned, but the press night was pushed back by a few days. So the cast was probably also pretty happy because Thursday night’s performance had all the trappings of a Broadway opening — enthusiastic crowd, press line, party — but no reviews looming later that evening. Those will hit the web Sunday night.