Richard Harris was perhaps the least likely candidate for the role of the Puritan leader who, according to many historians, carried out near genocide in Ireland. Although a fierce Irish nationalist, Harris saw past the historical circumstances and became intrigued with Cromwell as "a symbol of integrity, anxious to reform society", as the actor described him. Harris insisted it wasn't necessary for an actor to strictly believe in the character he was playing. Instead, Harris drew inspiration from Cromwell's idealistic nature, his goal to take the country out of aristocratic hands, and his "rigorous self-discipline", a trait Harris admired.
Cromwell died in 1658 at the age of fifty-nine. It is believed he died of malaria, and a urinary tract infection. In 1661, twelve years after the execution of Charles I, Cromwell's body was exhumed and posthumously beheaded.
More than two hundred workers at Shepperton Studios built the largest outdoor set ever constructed for an English-made movie, a two-acre re-creation of London's Parliament Square as it looked in 1642, complete with the House of Commons, Westminster Palace and Abbey, and roughly fifty other buildings.
Writer and Director Ken Hughes became hooked on the subject after reading a biography of Cromwell in the early 1960s. Over the next nine years, he read more than one hundred twenty books about him and toured England in his spare time between movie jobs visiting historic sites and conducting research in museums and record offices. Hughes was determined to pull together a tragic drama that would have "all the haunting inevitability of Greek tragedy." His dream became possible when he met Irving Allen, a producer who shared his obsession with Cromwell.