Al Pacino met a real-life mobster as part of his research to play Michael Corleone

He says in his book 'Sonny Boy' about struggling with getting to the heart of the part: "I still had to figure out who Michael was to me. Before filming started, I would take long walks up and down Manhattan, from 91st Street to the Village and back, just thinking about how I was going to play him.

"Mostly I'd go alone, other times I'd meet my friend Charlie Laughton downtown and we'd walk back uptown together.

"Michael starts out from a young man we've seen before, getting by, a little loopy, a little lumpy.

"He's there and not there at the same time. It's all building up to when he volunteers to take out Virgil Sollozzo and Capt McCluskey, the drug dealer and the crooked cop who conspired to kill Vito Corleone , Michael's father. All of a sudden, there's a big explosion in him.

"This is mapped out in the novel, because a book can give the narrative as much time as it needs. You wait and see how it unfolds. But what was I going to do in the film?"

Al added he finally got to grips with what to do onscreen after his 'Godfather' co-star Alfredo Lettieri - who played heroin trafficker Virgil Sollozzo in 85-year-old Francis' movie, took him him to a connected mobster's home.

He said: "Before we started shooting, I got together with Little Al Lettieri, who was going to play Sollozzo.

"He just said to me, 'You should meet this guy. It's good for what you're doing.'

"I sort of knew what he meant by that, so I went along with him. One day we took a drive to a suburb just outside the city.

"Little Al brought me to a traditional, beautiful, well-kept home. He took me inside and introduced me to the head of the household, a guy who looked like a normal businessman.

"I shook his hand and said hello, and he was very welcoming. He had a loving family. He had a wife who served us drinks and light snacks on fine porcelain. "He had two young sons around my age. I was just some crazy actor who had come into his house, trying to absorb as much as I could.

"Our conversation remained polite and at the surface level. I never asked Little Al why he had brought me here, but I thought about what he had said before we came, how this visit would be helpful for what I was working on. "Little Al knew some guys. Some real guys. And now he was introducing me to one of them.

"I was being given a taste of how this thing looked and operated in reality, not how it was shown in the movies.

"Not that our host was going to get into any of those details with us. As a matter of fact, we ended up drinking and playing games.

"Many moons later, photos from that night surfaced, showing me in a sweatshirt, laughing away with a drink in my hand, while Little Al showed me a gun. A boys' night out."