More, or Moore, was tried for High Treason for opposing the King's desire to be named head of the Church in England. Charges of treason were used to silence opposition to the king. When the Founding Fathers drafted the U S Constitution, cases such as More's led them to define treason in narrow terms, that is, "Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court."
Vanessa Redgrave had been due to play Margaret in the 1966 movie but had to withdraw due to commitments to the stage production of The Prime Of Miss Jean Brodie. She has a brief, uncredited cameo as Anne Boleyn instead.
In his autobiography, Charlton Heston says that he had hoped to be cast as Thomas More in the film version of Robert Bolt's play, but the part went instead to Paul Scofield, who had originated the role in the Broadway version.