Sam Briggs is upbeat, eager to please and, despite his best intentions, constantly stumbling into harm's way. No wonder proximity to Sam can be dangerous, especially for those whose approval he's courting hardest.

This is the premise for "Worst Week," a clever, rollicking new CBS comedy, premiering 9:30 p.m. EDT Monday, that begins on a fatefully awkward note. Sam and longtime girlfriend Melanie (Erinn Hayes) are expecting a child and planning to get married. But they must break this news to Melanie's parents (Nancy Lenehan and Kurtwood Smith), who already had serious doubts about Sam.

Adapted from British TV, "Worst Week" firmly plants itself on American soil, even as it steers Sam into comic quicksand. For instance, by the time he arrives for dinner at Melanie's parents' house, he is not only late but also clad in nothing but a makeshift plastic diaper — with his faith unshaken that he still can win them over.

It's the sort of thing that could happen to anybody. At least, it was when the evening began.

"We start with something that could plausibly happen, then push it to its limit," says Matt Tarses, the series' creator and executive producer. "We try to build in a way that feels organic, then take Sam to a heightened situation that explodes, creating new problems" — for him and everyone around him.

Tarses, who turns 40 the day before "Worst Week" debuts, was a producer on the celebrated "SportsNight" and "Scrubs." He says the biggest challenge in making "Worst Week" work was finding the right Sam.

Eventually the role went to Kyle Bornheimer. He's an affable, regular-Joe type with an unexpectedly resonant voice and rapid-fire delivery, who, at 32, has mainly been seen in a slew of humorous commercials that left viewers wondering, Who IS that funny guy? He jousted with a Stanley tape measure as his sword. He danced a little salsa for T-Mobile. He was assigned to mission control in a man's troubled bowel for Imodium.

He auditioned for "Worst Week" last fall, "but I was too stupid to hire him then," says Tarses. "He was good, but it was early and I felt like I might see someone else and —" as Tarses is talking, Bornheimer is seated across from him at breakfast during their whirlwind trip to New York.

"Were you pressed to cast a name?" Bornheimer asks him.

Tarses says yes, some of the suits preferred giving the role to a recognized actor.

"Then time went by, and around January one of Kyle's commercials was airing every five minutes and my dad called me and said, 'Have you seen this guy?' I said I had, and that he's really funny. My dad said, 'That should be the guy.'"

His father, Jay Tarses, is a comedy legend whose producing and writing credits include "The Bob Newhart Show," ''Buffalo Bill" and "The Days and Nights of Molly Dodd." (Sister Jamie Tarses is a former network executive and currently a co-producer of the comedy "My Boys.") As it turned out, CBS agreed with the elder Tarses' assessment.

With the show in production since early August, Matt Tarses praises Bornheimer for "throwing himself into everything with 100 percent of his energy and heart. Even when I write a kind of mediocre physical bit, he can make it into something more interesting than I imagined."

"If I'm exhausted and banged-up by Friday," says Bornheimer with a laugh, "then I know we've done our job."

The first few episodes will focus on Sam trying to get his future father-in-law to like him. The next half-dozen will feature the wedding. CBS has ordered 13.

And after that?

"I'm not at all over the mentality of a struggling actor, thinking all of this can be taken away at any moment," says Bornheimer, as if anything about him even hints at smugness, "but I'm excited when I think ahead to how Sam's world could continue to expand, and how he'll have other high-stress situations to deal with."

"We'll learn