Set during the summer months of 1953, Churchill is in his 70s and Prime Minister for a second time when hs suffers a life-threatening stroke, which is kept secret from the world.
Told from the viewpoint of his nurse, Millie Appleyard, it follows his battle to recover as his long-suffering wife, Clemmie, hopes the stroke will force Winston to retire while his political friends and foes scheme to plot who will succeed him.
“I’m terrible,” says Gambon. “I never do any research, although I did read through a book on Churchill.
“You look at him and see what shape he was, who he walked and all the sort of things that actors do.
“They you try to copy that as much as you can. I did become obsessed by the way he walked and the way he sat down. But ultimately, you have to do your own thing.
“People have said,’ You look like Churchill’, and I’ve thought, ‘Bloody cheek’. But I’ve looked in the mirror and there is a sort of likeness there. I’ve also tried to find a bit of his bloody-mindedness by making my voice bigger and not moving my top lip much.”
Churchill’s Secret, ITV, Sunday, 8pm