Upon his death, Michael took to social media to share a touching tribute to his father which explained the true dichotomy of the world's relationship with the Spartacus star and his own.
"To the world he was a legend, an actor from the golden age of movies who lived well into his golden years, a humanitarian whose commitment to justice and the causes he believed in set a standard for all of us to aspire to," Michael wrote, alongside a picture of his father from his younger years. "But to me and my brothers Joel and Peter he was simply Dad."
Through his own admission -- across a multitude of interviews and memories -- Kirk was a man who was proudly brash and commanding, with a larger-than-life spirit and strong personality.
"I’m a sonofabitch, plain and simple," Kirk famously stated in his 1988 autobiography The Ragman's Son. "I’m probably the most disliked actor in Hollywood. And I feel pretty good about it. Because that’s me…. I was born aggressive, and I guess I'll die aggressive."
According to Kirk, this aggressive attitude stemmed from a very unhappy childhood living in abject poverty with an alcoholic father who was physically abusive. The actor was very open about how the challenging experiences of his youth made him hard-edged and angry.
However, it also made him determined to not have the same kind of relationship with his own children. So, after welcoming sons Michael and Peter with his first wife, Diana Dill, he was determined to be a fixture in their lives. His divorce from Dill in 1951 made this more difficult, but Kirk tried his hardest to stay close to his young boys.
"I had sworn that I would never ignore my children the way my father ignored me," Kirk said in a feature interview with People in 1988.
According to Michael, he managed to keep that promise by visiting his sons, and having them visit him on set, trying to keep their relationship solid no matter what.
"We always knew he was there,” Michael told People in the profile on his father, "and that he really cared."
Even in 1988 -- when Kirk was 71 and Michael was in his mid-40s with two Oscars under his belt -- Kirk was still trying to serve as a source of guidance for his sons, with the point of view of someone who felt they had all the answers.
"To this day, Dad is the biggest advice giver in the world," Michael said at the time. "I tell him, ‘I’m in my 40s. It’s too late! Enjoy us or don’t see us. Relax! I’m not doing too bad!'"
Kirk, however, lovingly took credit for one of his son's most iconic roles, telling People, "I kept telling him he should play a villain. So finally he did, in Wall Street, and when he did that wonderful, horrible greed speech, I just [beamed]!"
It was his father's guidance, more through Kirk's dedication to his career and his craft than his actual words, that Michael credited for his own success and longevity after winning the Golden Globe for his role in The Kominsky Method in 2019.
"Stamina and tenacity,” Michael shared while speaking in the press room after his win, as to the lessons Kirk had imparted upon him over the years. "[My dad] was out of the school where you give your best shot, you give the best thing you can, and then f**k it. That was the best advice I got."
Throughout his career, Kirk remained one of his son's biggest supporters, and in his old age, their bond seemed to grow even closer.
In November 2016, Michael was finally honored with a star on the Hollywood Walk of Fame -- located near the corner of Hollywood and Vine, just a few feet from his father's star, which was presented to Kirk back in 1960.
In January, Michael and his wife, Catherine Zeta-Jones, spoke with ET on the red carpet at the 2020 Screen Actors Guild Awards -- where Michael was nominated once again for Outstanding Performance by a Male Actor in a Comedy Series for his role on The Kominsky Method -- and he opened up about how close he and his father were on a daily basis.
"We had dinner with him last night," Douglas said. "We got him an iPad, so he can Facetime us, and now he calls religiously."