The words "Man of Principle" should float over Thomas More's head in colorful balloon letters… after all, that's what he is and that's what A Man for All Seasons is all about—sticking to your guns even when someone is (literally) threatening to chop your head off.
To recap the basic plot points: King Henry VIII wants to break with the Catholic Church and the Pope in order to secure a divorce and marry his mistress, Anne Boleyn. He and his supporters see this as essential in order to produce a male heir and prevent civil war. But Thomas More, the Chancellor and one of Henry's most trusted advisers, breaks ranks and refuses to take the oath approving the king's remarriage. His silence is considered high treason and he is put to death by beheading. (Yeesh.)
But in case you think Thomas More is all principles and no fun, the filmmakers make sure to color in More's human side. When we first see him, More is hanging out and laughing with family and friends—he's just a dude.
By contrast, the cynical guys who support Henry's divorce, like Cromwell and Richard Rich, are the ones who end up seeming cold and fanatical. Compared to their machinations, More's martyrdom feels like a warm and natural action. Ironically, it's the future martyr who knows how to have a good time; his opponents are too wrapped up in their own power games to notice when the sun is shining. (Note that it's a beautiful day when More gets executed—flowers are blooming and birds are chirping.)
We see More in this initial state of family happiness with his beloved wife Alice and their daughter, Margaret, whom he has trained to be a scholar (she speaks Latin better than the King—burn). But the events that will rupture this happiness quickly pile on. A trip to see Cardinal Wolsey, who's trying to secure the king's divorce, and a visit from the king himself lead More to realize that he can't remain faithful to his Catholic principles and the king's wishes at the same time.
He needs to choose: If he puts his chips in with the king, he retains his power and status as the king's chancellor… but loses his conscience and, in a sense, his sense of self. If, on the other hand, he sides with the Pope, he puts his own life at risk and leaves his family to fend for themselves… while remaining faithful to what he really believes.
Ultimately, he sides with his faith and loses his head. Literally: His noggin is chopped off.
Our Tom believes wholeheartedly (and wholesouledly) that it's not the devil that's in the details… it's God that's in the details. To be a true believer, you have to believe in all the nitty-gritty rules of the Catholic Church.
For More, the important thing isn't the specific person who happens to be Pope—it's what the whole institution of the Church represents, which is the reign of Christ as delegated through the Apostle Peter, the first Bishop of Rome.
If King Henry breaks that chain of descent from Christ and stops bowing to the Church's authority, More believes that he will be creating a false church. To More, his belief in the Catholic Church's rightful, God-given supremacy goes so deep within him that he views it as part of his very self. If he were to side with the king, he wouldn't just betray the Pope—he would betray himself and he would betray God.
He makes this pretty clear, telling his old friend, the Duke of Norfolk,
MORE: Affection goes as deep in me as you, I think. But only God is love right through, Howard, and that's my self.
He also tells his daughter,
MORE: When a man takes an oath, he's holding his own self in his own hands like water. And if he opens his fingers then, he needn't hope to find himself again. Some men aren't capable of this, but I'd be loathed to think your father one of them.
Thomas More is a true believer. He goes all in… and he's in it to win it.
The movie tries to demonstrate that, while More could potentially be portrayed as a fanatic (for example, Hilary Mantel's Booker Prize-winning Wolf Hall makes Cromwell look sympathetic and More appear jerky), his move toward martyrdom wasn't the act of someone who actually wanted to die.
More is always alert to the possibility that he might be able to find a way out—he never openly denies that the King has the right to break with the Church until after he is convicted at the very end of the movie, and he hopes that he can take the oath without violating his principles, though he discovers that the wording of the oath makes it impossible.
More isn't glad to see his wife and daughter suffering thanks to his imprisonment—it pains him worse than torture would, he says. In a way, their love tempts him to renounce his principles far more than Cromwell's conniving could.
Just before More dies, he forgives the executioner and has a brief exchange with a priest:
MORE (to the Executioner): […] I forgive you, right readily. Be not afraid of your office. You send me to God.
PRIEST: You're very sure of that, Sir Thomas?
MORE: He will not refuse one who is so blithe to go to Him.
At this point in the story, More is indeed totally blithe to go to God. But the road to that place of acceptance was a long and painful one—which might've made More's death feel like a definite release.
(More was played by Paul Scofield, who won a totally deserved Best Actor award at the 1967 Oscars for his portrayal.)