Walters, who is known for starring in seven of the eight "Harry Potter" films as Molly Weasley, told the BBC that she initially thought doctors had "made a mistake" when they instructed her she had stage 3 bowel cancer weeks before she was supposed to attend the 2018 premiere of "Mamma Mia! Here We Go Again."
"I was still thinking, 'That's ridiculous, he must have made a mistake,'" Walters said. "I couldn't believe it."
Walters, 69, told the BBC that she disclosed her diagnosis to family members and friends, but had never spoken about it publicly until this most recent interview.
"I'll never forget his face," Walters said, recalling when she told her husband, Grant Roffey, about the doctor's appointment. "Tears came into his eyes and I thought, 'Oh, Grant!' So then I'm worried about him more than [the diagnosis]."
Walters had part of her colon removed and underwent chemotherapy. Although she has recovered from bowel cancer, she said the diagnosis inspired her to reevaluate her workload.
The Academy Award-nominated actress is also known for her roles in "Billy Elliot" and "Mamma Mia." Walters will also star in "The Secret Garden," which is set to premiere this spring. She said it's possible that this role may be her "last," but is open to the possibility of postponing her retirement.
"But, you know, something might come up," Walters told the BBC. "It would have to be something I really engaged with and that didn't have a killing schedule."
Walter's diagnosis has also encouraged her to urge others about the importance of getting tested for bowel cancer, even if doing so may be uncomfortable.
"Your bowel is part of your digestive system. It's just what digests your food and I think you just have to remember that," Walters said. "And it's just, you've got to go and get things checked. Doctors are used to bottoms: They've got one themselves, hopefully."
Bowel cancer is the third most common cancer in both the United Kingdom and United States. The American Cancer Society estimates that more than 100,000 people in the U.S. will be diagnosed with bowel cancer this year.