Bob Newhart on How Mary Tyler Moore Hid Her Private Pain

Bob Newhart has the utmost respect for late actress Mary Tyler Moore, who passed away Wednesday at the age of 80.

Although the Emmy-winning star faced numerous physical and emotional battles throughout her life — including diabetes, alcoholism and the death of her son — fellow actor Newhart, 87, remembers the Mary Tyler Moore Show leading lady as only being cheerful and positive.

“She lost her son. I never saw that side of her — I never saw her when she wasn’t up and smiling. She never let show,” the Bob Newhart Show star tells PEOPLE about the death of Moore’s son, Richie, who died of a self-inflicted gunshot wound at the age of 24. “She had it tough, but boy you’d never guess it. She was always happy and bubbly. You’d never know anything was going on.”

“Given the hand she was dealt, she was amazingly positive. I can’t imagine what losing a son would be like,” adds Newhart, who met Moore when she starred on The Dick Van Dyke Show from 1961-66.

Newhart, who last spoke with Moore three to four years ago, also praises the actress for her dedication to her work and the strides she made for women in the workforce.

“She loved to work. She loved to rehearse. She was very, very professional. It showed in her work,” he said of Moore’s career on-camera. “Mary Tyler Moore was always behind you. If you believed in something, they would fight for it. Mary’s show was quite revolutionary for its time. Her role – she was this independent woman who had just moved to Minneapolis. She was on her own and dating … It was quite revolutionary in it’s time.”

Throughout her life, Moore lived through two troubled marriages and struggled with alcoholism, which she chronicled in her 1995 memoir After All. A second book of hers, 2009’s Growing Up Again: Life, Loves, and Oh Yeah, Diabetes, later addressed her disease.

“She handled it incredibly. I don’t think I was aware of the alcoholism. I knew about the diabetes and of course the divorce was painful,” says Newhart, who admits that he was “shocked” when she divorced from Grant Tinker: ”You always pictured them together forever. But television is rough — it’s a demanding mistress.”

But despite the many personal struggles that she faced, Moore continued to remain positive — and private — throughout her life.

“That’s the way Mary handled things. You didn’t know,” says Newhart. “She handled them very privately. She had a lot going on.”

Moore is survived by her husband of more than 33 years, Dr. S. Robert Levine.