The veteran Irish actor was concerned because he disliked Churchill and had once played Michael Collins, his adversary.
"I didn't know if I was going to be able to do it or whether it was a bridge too far," he said.
"I wasn't sure what my relationship with this character could be. So, we did a camera test at the outset –for both me and the producers, to see whether me playing Churchill was just a daft notion.
"My voice coach was my savior. She came over and worked with me for two weeks.
"At the beginning of the first week, she asked me whether I was looking forward to the role. I said: 'Absolutely not! I have a ball of dread in my stomach and I don't think it will go away and I don't believe a word coming out of my mouth.’
"By the second week I had a little run where I thought, 'Actually I kind of believed that,’ the real test was whether I was able to get hold of Churchill's private intimate voice. It had to be perfect otherwise it wasn't going to work. That's where the terror was."
Gleeson said he had to shave his head and gain weight, which he says he “never lost, which was kind of irritating.”
Back in 1991, the Irishman played Michael Collins, who had fought bitterly with Churchill in the run up the Anglo-Irish treaty of 1921.
"I knew how it felt to be on the other side of Churchill's wrath. I was wondering, 'Why am I taking this role?' I knew I'd have to shed some baggage if I was going to do it honestly," Gleeson confessed.
"It was very interesting having played Collins who was sat facing Churchill across the table in The Treaty and then just to switch chairs. It's like an old acting exercise, where people have to switch roles.
"When you're in someone's corner, you've got to fight for them. I had to leave behind some pre-conceptions."
Despite all that he says, the actor’s respect for Churchill grew.
"His faults really were his strengths and his strengths really were his faults. He had a bull-headedness and determination to push things through,” he said.
"If you're on the end of it, it doesn't feel too good. If he's actually stopping a savage like Hitler, it's pretty fantastic to have him on your side.”