From Radio to Stage to Screen
Not many texts jump through not one, not two, but three media… and get awarded a Best Adapted Screenplay Oscar at the end of it all.
Robert Bolt first wrote A Man for All Seasons as a radio play, which eventually developed into a Broadway/West End smash. (It was the Book of Mormon of its day… sort of.) When it came time to adapt it into a movie, Bolt took on the writing duties, authoring the screenplay himself.
He cut out some parts from the play—for example, originally, there was a character called "The Common Man" who would directly address the audience and play different minor parts in the play. The Common Man isn't exactly a bad guy, he just represents a "keep your head down, do your job" attitude, contrasting with More's martyrdom. Anyway, Bolt cut the Common Man and added a few minor scenes.
If you look at the other achievements Bolt is known for, you start to see a pattern. Critics have pointed out that Bolt's other famous screenplays deal with courageous individuals who don't fit in. Bolt worked with the director David Lean on his classic movies Lawrence of Arabia and Dr. Zhivago, both of which deal with courageous individuals who are set apart—outsiders in some way (source).
Lawrence, Zhivago, Thomas More—they're all nobody's trained seal, and they all go their own way. Interestingly enough, Robert Bolt wasn't a religious believer, even though A Man for All Seasons exalts a Catholic saint and martyr. (In fact, Bolt was a former member of the British Communist Party and an agnostic.)
Nevertheless, he felt that More's commitment to his principles had some relationship to his own aspirations (Bolt was an activist in the nuclear disarmament movement, for one thing). Yet, he didn't draw any specific or explicit parallel between his own activism and More's stand—he wasn't comparing himself to More. Instead, he was drawing a depiction of a "Man of Principle" to whom anyone who was attempting to lead a principled life might relate.