The film Becket has its flaws; not least the fact that the title leaves out the word 'saint', meaning that someone could think that the film was about the Irish playwright Beckett. The film places a lot of emphasis on King Henry and Becket's reputations as being wild lads who seduced simple barely-able-to-speak peasant girls.
But the triumph of the film is its colourful portrayal of Henry II's persecution of his dear friend St Thomas Becket. Peter O'Toole in his role as Henry II is the eyes-darting, much-younger-than-Becket king who resents that Becket's vocation to the priesthood has distracted him from being his close pal, and that Becket's allegiance to the Church comes first. At times it's as though Henry II wanted the Church to serve him, rather than he serve the Church. Henry II has other reasons to be jealous - he shakes with anxious envy when he hears accounts of Becket's charisma and his ability to effortlessly draw adoring crowds and command respect without being a tyrant. The film captures a fine sense of Henry II, who preferred Becket to his wife and children, and who was mad with grief for the loss of his friendship with Becket, not least the fact that Henry felt betrayed and embarrassed that Becket refused to obey him and rebelled against the injustices of his reign. Some would say that Peter O'Toole overdoes the role of the immature monarch who fiendishly chastises Becket because the saint had outgrown their boyish, high-jinks friendship. Many consider the scene in the clip below - when Henry II attempts to excommunicate Becket - to be the best in the film.
Read a very good and well-written account of St Thomas Becket's life here.