John Gielgud received an Oscar nomination for appearing in just two scenes.
Richard Burton claimed to have been offered either of the main roles. However, according to the producers this was not true, since Peter O'Toole had already been cast as Henry II and Becket had to be the older man. Burton was seven years older than O'Toole.
Richard Burton initially turned the film down because he felt the idea of him playing a saint would cause the press to have a field day. He also said he would be more suited to playing Henry II.
Hal B. Wallis was not at all happy with Peter Glenville's choice of composer for the film as he'd never heard of Laurence Rosenthal. However, Glenville was adamant, having worked with Rosenthal on the stage version of the play.
Alec Guinness turned down the role of King Louis, because he "didn't believe in Becket as a film".
Peter O'Toole played the same character, Henry II, four years later in 1968's The Lion in Winter. He received Oscar nominations for both films.
Peter O'Toole's crown was made of cardboard.
A point not mentioned in the play is that Queen Eleanor's first marriage had been with King Louis VII of France; the marriage was later dissolved for reasons of common ancestry, although Henry was exactly as close a relation as Louis had been.
As a practical joke, Peter O'Toole had Richard Burton's then wife Elizabeth Taylor take the place of a nude actress lying under a blanket in order to surprise her husband. Burton was not amused.
Becket's tomb became a popular destination for pilgrimages for centuries, but it was looted and destroyed in the early 16th century in the Dissolution of the Monasteries during Henry VIII's reign, as part of the Church of England's break from Rome. A shrine was later created at the site.
Both Peter O'Toole and Richard Burton were on their best behavior during filming as they were in company of Donald Wolfit and John Gielgud, respectively O'Toole and Burton's acting mentors.
Composer Laurence Rosenthal got a hard time from producer Hal B. Wallis who insisted on a more upbeat score. Rosenthal held his ground - and was rewarded with an Oscar nomination.
Empress Matilda (Maud), King Henry's mother, had been chosen by her father King Henry I to rule after his death; but the ruling Council of England decided it would be inappropriate for a woman to rule, and named her cousin Stephen as king. This set off decades of war, during which Matilda captured much of western England and was proclaimed Lady of England. Though she never became queen, she successfully established her son Henry's claim to the throne. She died in 1167, three years before Becket's murder.
Final film of Frank Pettingell.
Final film of Sir Christopher Rhodes.
It is often alleged by various who's who's of the cinema that Ronald Lacey has a role in this film but he is nowhere to be seen.
King Henry actually was very close with his son Henry, choosing to have him raised in his close friend Becket's home - a common practice for royalty in that era. The prince's resentment over his father's possible causation of Becket's death (intentional or not) was a major reason he became distanced from his father in later years, although the prince eventually died six years before his father.
Most of the film was shot in sequence.
One of the main musical themes in the film is an adaptation of a Welsh folk tune, which Siân Phillips taught composer Laurence Rosenthal.