Richard Burton initially turned this movie down because he felt the press would have a field day over the idea of him playing a saint. He also said he would be more suited to playing Henry II.
Richard Burton said he was offered either of the main roles. According to the producers, Peter O'Toole had already been cast as King Henry II, and Thomas Becket had to be the older man. Burton was seven years older than O'Toole.
Richard Burton and Peter O'Toole were considerably younger than their characters were at the time of the events depicted in this movie.
The re-creation of Canterbury Cathedral, built on Shepperton's Stage H and designed by John Bryan, was the largest single interior set built in Europe up to that time.