ET is on the set of Harry Connick Jr.'s Lifetime Original Movie, "Living Proof," and the actor candidly speaks about the pain of losing his mother to cancer -- a subject matter brought on by his new acting role.

"I do know how I responded to her death, and it was by far -- with the exception of, God forbid, losing a child -- the most painful thing I'll ever have to go through," Connick reveals to ET.

In the movie produced by Renee Zellweger and the executive producers of 'Chicago,' Craig Zadan and Neil Meron, Connick plays a doctor who is testing an experimental new breast cancer treatment drug.

The centerpiece in Lifetime's annual public awareness campaign, "Stop Breast Cancer for Life," 'Living Proof' tells the true story of the UCLA doctor, Dr. Dennis Slamon (Connick), who helped to develop the breast cancer drug, Herceptin. Aided by his research assistant (played by Amanda Bynes), the film follows the good doctor as he endures the massive trials and tribulations in order to keep the drug testing afloat. Despite being met with skepticism by his colleagues and the drug company funding his research, Dr. Slamon finds help from others and prevails in the face of adversity.

Connick joins a star-studded cast that includes Angie Harmon, Swoosie Kurtz, Bernadette Peters and Amy Madigan, who help paint the picture of the women afflicted with the deadly disease, and of the man who battled against all odds to save those lives and thousands more.

The Lifetime film premieres Saturday, Oct. 18 on Lifetime Television.