Not working together again, was more on Marlon, due to his eclectic acting choices, along with an increasing reluctance to continue acting at all, in a profession he felt contributed little to a world that had more important issues to address. As for the two actors working relationship, Marlon actually took it upon himself to look out for Al Pacino, who was not making anyone happy with his non-showy portrayal of Michael Corleone during filming, including Director Coppola, & was close to being replaced; this excerpt is from a 2010 interview with Pacino:
“To work alongside Marlon Brando in a movie was initially a little unnerving to me. The studio heads were trying to get me fired. I knew how I wanted to play the role, but no one could see what I was doing. I didn’t want to be where I wasn’t wanted and it became a very difficult time for me. Marlon took me under his wing and told me he was really happy with my performance. He said, “I want you to know that I really want you for this part.” He defended me to Francis and the studio, saying, “The kid is doing great.”
The way Marlon was and the way he treated me, he was very sensitive and good to me. His support was invaluable to both my role as Michael Corleone and to my own self-confidence as a young actor. I loved him. Acting is my life, and through it, I have discovered an understanding of myself and a way to express my innermost feelings. I am grateful to have known Marlon Brando, not just for his kindness to me during the filming of ‘The Godfather’ but for the opportunity I had to act alongside him in witness to his great talent. “