The Harry Potter star was struck down with acute leukaemia in 1996 but vowed to complete his duties shooting a TV advert before starting treatment.
But medics urged him to attend hospital immediately, revealing he was so ill he was close to death.
Spall's wife Shane detailed their agony over his health battle in her diaries, which have been made into a new book, The Voyages Of The Princess Matilda, and serialised in the Daily Mail.
She tells of the terrifying moment Spall was diagnosed with the disease and his desire to carry on working despite his illness, writing, "We'd been in the garden one day in April 1996 when you stood in front of me with fear in your eyes. You opened your mouth, and said: 'Look at these.' There were horrible purple-red patches on the inside of your mouth. 'Phone Dr Gaynor,' I'd said. 'He'll sort you out. You're run-down, darling, you've been working too hard.'
"After you'd seen the doctor, you left a message for me at my gym. 'Can you come? I've got a serious blood disorder.'... Our doctor says to get you to University College Hospital in London now, but you won't let the crew of a TV advertisement you are filming down. You say: 'I'll be finished in a couple of hours - go home and pack me a bag, I'll only be in hospital overnight'... Dr Panos replied: "You're lucky to be here. You have 90 per cent leukaemic cells.'
"On the phone, your doctor said to get you to the hospital immediately, in case you had a brain haemorrhage. Apparently, he said, you could drop down dead at any moment. You're facing a life-threatening illness, but you want to finish the job first."
Spall eventually went into remission and was given the all-clear in 2002.