IF YOU can rely on anyone to portray Sir Winston Churchill - and do it justice - it's the graceful Sir Michael Gambon.

Best known for his role as Albus Dumbledore in the Harry Potter franchise, the veteran actor, 75, portrays the former Prime Minister in ITV's upcoming feature length film, Churchill's Secret.

Set during the summer of 1953, Gambon's depiction will see Churchill – now in power for the second time and in his late 70s – suffer a life-threatening stroke, which is kept secret from the world.

Based on Jonathan Smith's book, KBO: The Churchill Secret, the adaptation is told from the viewpoint of his young nurse, Millie Appleyard (Romola Garai), and follows his battle to recover as his long suffering wife Clemmie (Lindsay Duncan of Rome fame) desperately hopes the stroke will force him to retire, while his political friends and foes scheme to plot who will succeed him.

It's often said that, above all else, preparation is the key to success - unless you're seasoned performer Gambon, it seems.

"I'm terrible. I never do any research, although I did read a book on Churchill," admits the Dublin-born star.

"You look at him and see what shape he was, how he walked and all the sorts of things that actors do. Then you try and copy that as much as you can. That's how it ends up; you just hope and pray you're doing it properly.

"I then found myself in a rehearsal room with the other actors who all happen to be friends of mine. I couldn't believe it - I thought they'd all been arranged especially for my benefit."

The cast also includes Bill Paterson, James Wilby, Alex Jennings, Patrick Kennedy, Christian McKay, Chris Larkin and John Standing.

With a career that spans five decades in theatre, TV and film, Gambon is a master of his trade but, he reveals, his nerves have increased with age.

"It's very frightening, acting. The older you get, the better you get, but the more frightened you become. I do, anyway. It's frightening playing someone as famous as Churchill. You can play King Lear, because no one knows anything about him.

"Every day, I turned up and played Churchill as best I can. I'm obsessed by the way he walked and the way he sat down, but you've got to do your own thing."

As for any physical likeness, he's still not sold.

"People have said, 'You look like Churchill', and I thought, 'Bloody cheek'. 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."

Focusing on the summer of 1953, during which the leader - nicknamed 'The British Bulldog' - suffered two strokes, Churchill's Secret unravels the truth behind his closely guarded health.

Speaking of the cover up, Gambon, who was 24 when Churchill died, says: "His illness was kept secret and it left the country without a leader. We couldn't do that now.

"Luckily he recovered, and the public never knew about it until after his death. Many people watching this film won't know about this section of his life."

Filming at the statesman's once-home, Chartwell in Kent, it also documents the relationship between Churchill and his wife - and life peer in her own right - Clemmie.

"It was a bit on and off and a bit rough sometimes, as with all marriages," Gambon shares. "But fundamentally, he loved her. We bring that out in this film; some of the scenes are quite moving."

Matter-of-fact about growing older ("There's nothing you can do about it. You just take it, don't you? What can you do?), father-of-three Gambon is happy to dismiss the age tag. He confesses he still enjoys a spot of fan attention, however.

"From Harry Potter I get noticed, but that's all right; it's good fun. But it's getting thinner. The mail from Harry Potter when I was in it and when it was running would be piled high every week - now it's down there a little bit. It's a bit upsetting, really."

That's not to say his days of being recognised are over though...

The much-loved star recalls "nosy" people looking through restaurant windows when he was filming Dad's Army and, more recently, peering over the hedges at Chartwell.

"That was quite nice, actually," he says. "I signed a few autographs, but I wasn't mobbed. They're not like that. They're quite posh, the people who go to Chartwell."

Having spent nearly 50 years on the stage across London and the US, Gambon declares that while his famous decision to call time on his theatre career last year was necessary, it's left him feeling dejected.

"I had to. I can't remember the lines," he says.

"It's a fact of life but it's a shame; I feel heartbroken. I went on the stage recently with a mate of mine, to try and see if it would work if I put a plug in my ear and got someone in the wings to prompt the lines, but it won't. The theatre is about speed of delivery and quickness of reaction, and it wouldn't fit in with that."

But with a busy 2016 ahead - he's currently filming biopic Mad To Be Normal and has several projects in pre and post-production, life doesn't look set to slow down for this national treasure.

"I'd be mad to retire," Gambon agrees, chuckling. "I've been acting for over 50 years; I wouldn't want to stop now. There'd be no reason, unless someone stops me.

"I keep rolling on. I like it."

Churchill's Secret is on STV on Sunday, February 28