Marg Helgenberger is saying goodbye to "CSI"

Long-running TV series "CSI:Crime Scene Investigation" will take on a less serious tone this season as former "Cheers" actor Ted Danson joins the cast of forensic detectives, producers said on Wednesday.

But in a surprise announcement, lead actress Marg Helgenberger told reporters that she plans to leave at the end of the upcoming 12th season of what CBS says is the most watched TV series in the world.

Helgenberger, who plays former CSI supervisor Catherine Willows, said she had considered leaving at the end of last season, but decided she was "having a hard time letting go and that I was not yet done playing this character. That's why I decided to extend (my contract) a little bit."

Danson, 63, takes over from the departing Laurence Fishburne (Dr. Raymond Langston), playing a new character D.B. Russell -- a family man with four children, an unconventional hippie upbringing, and a good sex life.

Executive producer Carol Mendelsohn said Russell has his own unique process for solving crimes, adding that "the season will be a little lighter. There is more humor than there was last season.

"Season 12 -- that's a lot of years, and we wanted to excite the audience and excite ourselves as writers," Mendelsohn added of the change of tone.

"CSI" is one of the most valuable franchises at CBS and is licensed in 200 countries. But U.S. audiences have fallen by about four million viewers in the past two years since original headliner William Petersen left the show in 2009 and Fishburne took over, bringing a far more serious edge to the series.

CBS entertainment chief Nina Tassler acknowledged that Peterson's departure had been "impactful". But she said that Danson brings a huge fan base and could attract new audiences for the series.

"It is a successful show, long running, and I think it is a great opportunity for us," Tassler said.

Emmy-award winning actor Danson -- best known as the skirt-chasing bar owner Sam Malone on the 1980s comedy "Cheers" -- said he was surprised but pleased at the chance of returning to a leading role on a top-rated network TV series.

"I was thrilled, happy... My jaw is still hanging down a little bit from the newness of all this.

"I love going to work. I am so happy to hang with this cast...to be able to step into that atmosphere is a joy," he said.

Danson also revealed another, little known qualification for his new role. "I grew up around skulls. My father was an archeologist in Arizona and I would go on digs with my dad as a kid."

Danson makes his debut in the new season premiere on September 21.