The Harry Potter star was filming a commercial in 1996 when he was diagnosed with acute leukaemia and ordered to attend a hospital for emergency treatment.
The stunned star fell to his knees and begged for divine intervention - and his prayers were seemingly answered when aggressive chemotherapy put the disease in remission.
Spall tells U.K. TV show This Morning, "I said (to the doctor), 'Tell me what's going on. What's happening?' He said, 'Well old boy, I'm sorry to tell you, you have got leukaemia'. I said, 'I'll phone you back'.
"I fell on my knees and I prayed to God - 'Please don't let this be it'. And I phoned him back and said, 'Can I have a drink?' and he said, 'No, your brain might explode!'"
Spall, who was given the all-clear in 2002, also reveals he and his wife Shane were determined to keep the details of his health battle a secret from fans.
He adds, "We kept each other's pain from each other. I knew she was in hell, and she knew I was in hell, but we never said we were in hell. I just got on with it... I never, ever wanted to be known for anything other than being an actor... I just wanted to keep it quiet so we could get through it on our own."