"I couldn't believe it," she revealed to Derbyshire.
Walters also says she'll never forget the moment she told her husband, Grant Roffey.
"I'll never forget his face. And tears came into his eyes," she said.
The terrifying experience has "completely changed" her perspective towards acting.
"I feel like the person before the operation is different to this person," said Walters, also adding that she is grateful she has taken a step back from her insane workload.
When asked if The Secret Garden could be her last film, she said it was possible.
"It would have to be something I'm really engaged with [to take another role on].
"I'm not saying I'll never act again. But I certainly don't think I can go back to [a film that requires working] six days a week, five in the morning till seven o'clock at night."
Walters has been nominated for two Academy Awards — for her supporting roles in Educating Rita and Billy Elliot — and also played Molly Weasley in the Harry Potter movie franchise.