I went to Venice to persuade Mastroianni to do my movie, Leo the Last. I found him sitting with Faye Dunaway on the terrace of the Gritti Palace Hotel. […] Tourists on gondolas and water buses glided past, heads turned in awed unison at the sight of a Latin legend and an American screen goddess. Marcello wore the bemused but resigned air that we know so well from his movies, the one that says, “I have no idea
why people think I am a movie star, or why beautiful women want me to make love to them, but since it is so, it would be churlish of me not to oblige”. I came to realize that he was that rare creature, an actor without vanity, self-deprecating, yet completely at ease with himself, and this was the fountain of his effortless charm, a charm that included and beguiled all who came into his presence. He came to London and we made the movie. […] Marcello would arrive in the morning no more conscious than a side of beef. His dresser, Fred, would administer a cup of the severest espresso which he drank while putting on his costume. With the clothes he also put on the character, Leo. It was total transformation.
He stayed that way throughout the day. I could have filmed him in his lunch break. At six, he put on his own clothes and shed the character he was playing and never gave it another thought until the espresso hit him the following morning. He was like a factory worker, toiling contentedly but always glad to hear the whistle. […] He was a dream for a director. He could make any scene work. A twitch of an eye, a hunched shoulder, could render lines of dialogue redundant. Technically, he could manoeuvre himself through the most awkward moves to solve a camera problem. I would show him a complicated series of marks I wanted him to hit. He would shrug. No problem. […]
He did not insist, he did not protest, he did not plead; he was simply present. I never felt the need to apologize to Marcello for making him wait. I had a scene where he was sleeping in bed. While we were lighting and lining up, he simply fell asleep in the bed. I had to wake him up, so that he could act being asleep.

John Boorman, Remembering Marcello, in Projections 7: filmmakers on film-making, John Boorman and Walter Donohue (ed.), Faber and Faber, London-Boston 1997