Lara Flynn Boyle Lara Flynn Boyle

Here is one secret that Sin City can’t keep: Lara Flynn Boyle is back. The woman who survived rampant rumors of anorexia, a public firing from ABC’s The Practice and an eyebrow-raising romance with Hollywood heavy Jack Nicholson will heat up the Strip this fall as a new costar on NBC’s Las Vegas.

Boyle will play Monica Mancuso, a billionaire widow who takes over the newly renovated Montecito (the old casino blew up in the Season 2 finale). "She’s the mystery buyer from last season," reveals Las Vegas creator Gary Scott Thompson. Adds Boyle, "She is a woman with a wad of cash. Only Monica bought a casino [with it] instead of a lot of shoes."

Boyle plans to look to the real-life exploits of Shirley MacLaine to shape her character. "I always daydreamed about being one of the Rat Pack," says the actress. "Shirley MacLaine could really hold her own with [Frank Sinatra and] the boys. I feel the same way — never let them see you sweat or cry."

Not that the 35-year-old amateur Irish folk dancer (no, really!) ever could be mistaken for the crying type. After Practice creator David E. Kelley fired her for "creatively driven" reasons in 2003, she blasted the move in the press.

When Las Vegas returns Sept. 19, don’t be surprised if romantic sparks fly between Boyle’s and James Caan’s characters — even though Big Ed is still married (Cheryl Ladd, who plays his wife, will return). "I wouldn’t put it past Monica," Boyle says. "The purpose of adding her to the cast is to wake people up."

Also making the move to Vegas: Rachael Leigh Cook, the former teen model whose breakout role was in the 1999 comedy She's All That. But will the Montecito's security chief agree with that assessment? Cook has been tapped to play a real-estate agent who catches Danny McCoy’s (Josh Duhamel) eye in four episodes. The demoiselle originally was slotted to costar opposite Chris O’Donnell in the upcoming Fox dramedy Head Cases — in fact, she taped the show’s pilot — but her role was recast after test audiences insisted that Cook, 25, looked too young to play O’Donnell’s love interest.