The execution of King Charles I was filmed on the exact same day the actual execution took place some 300 years earlier.
The films turbulent production was constantly beset with financial problems & funding went bankrupt twice during filming. the film was only barely completed & eventually released through private investment.
Rupert Everett studied Alec Guinness playing the same character (King Charles I) in the film Cromwell (1970) and used Guinness' way of portraying the stuttering of the king as basis for his own performance.