The secret to a successful career, according to Morgan Freeman

At 80 years old, Morgan Freeman is as active as he's ever been in film and television.

From his numerous upcoming film credits, to his role as the executive producer and star of National Geographic's new docu-series "The Story of Us," Freeman keeps a hyperactive work schedule that would find most other octogenarians — or any working-age adult, for that matter — in a consistent state of fatigue.

In a recent conversation centered on "The Story of Us," Freeman told Business Insider about how his approach to self-discipline has allowed him to achieve longevity in his life and career.

Freeman's passion to enlist in the US Air Force as a teenager appears as a touchstone throughout "The Story of Us," and his views on discipline have a certain military-like strictness to them.

When asked what his secret to longevity has been, Freeman responded, "Discipline," and then elaborated:

"Exercise, part of your discipline. How you eat, part of your discipline. I try not to overeat," he said. "One of the things that I discovered somewhere back down the line was that eating, for us particularly here, has become a habit, not necessarily a need. So if you try to keep it down to need, it's going to be much better for you."

He went on to contrast American society's current dietary habits with the "time in history" when obesity was rare and more physically demanding labor made eating a form of fuel.

"You know there are more obese people in the US than probably anywhere else? Because we can feed them," he said. "In the time in history when everybody had a job, an actually physical job to do — you get up in the morning, and you get your hoe or your axe or your saw, or whatever the tool it is that you're using, and you use it. And then at noon, you stop using it and refuel. And then you use it some more, and then you go home, and you refuel.

"Now, let's say you get up in the morning, and you brush your teeth, you comb your hair and put on a suit," he continued. "And you go and sit down at a desk. You haven't used up anything, comparatively."

Freeman's six-episode series, "The Story of Us," finds the actor traveling the globe to interview a diverse multitude of people, including famous public figures like Bill Clinton and Nadya Tolokno of Pussy Riot.

In our interview, Freeman described the series as an attempt to "on some level, reduce the amount of tension between people who don't know each other."