Behind The Scenes Of 'CSI'

The seventh season of "CSI: Crime Scene Investigation" has been a tumultuous one. So far, investigator Greg Sanders has been nearly beaten to death while Catherine Willows not only dealt with her daughter's kidnapping but also with her father's murder.

"I've had a fun time, actually," Marg Helgenberger, who plays Catherine, told The Early Show co-anchor Julie Chen. "Some people say to me, 'God, weren't you exhausted shooting those two episodes?' And I said, 'Eh, no, it actually was energizing.' "

It's not only a tough job emotionally, the logistics of shooting "CSI" can also take a toll. Recently, the crew filmed two episodes at the same time. Moreover, "CSI" has been in a back-and-forth ratings battle this year, pitted against the hospital drama "Grey's Anatomy."

"Certainly I think we've brought our competitive game," said Jorja Fox, who plays Sara Sidle. "I think that we still definitely want to win, you know. I want to win. Who doesn't want to win?"

All the hard work pays off every Thursday. In tonight's episode of "CSI," twin sisters are killed within hours of each other. Grissom, played by William Peterson, and Catherine see the case in two different ways: Catherine thinks the deaths are related but Grissom is not so sure.

For a few episodes later this season, the "CSI" creative team will have to deal with the absence of Petersen, who is taking time off to do a play in Rhode Island. Grissom will be gone, teaching an out-of-town workshop. During that period, he will be replaced by acclaimed actor Liev Schreiber.

"It's gonna be a little different around 'CSI,' so that'll be good," Peterson said.

Schreiber's first "CSI" appearance comes early next year. The show is keeping it a mystery how many episodes he'll be on. "CSI" airs tonight at 9 p.m. Eastern and Pacific, and 8 p.m. Central.