Dame Helen Mirren and Nicol Williamson were initially reluctant to work with each other. They'd been in a disastrous production of "Macbeth", and were not on speaking terms. Writer, producer, and director Sir John Boorman cast them because their natural animosity would be perfect. According to Mirren, she and Williamson "wound up becoming very good friends" during filming.
The initial fight scene had to be filmed three times. It was filmed at night, and all of the film came out underexposed the first two times, due to a fault in the exposure meter. The cameraman had a nervous breakdown over the issue and quit.
Morgana's breastplate is in writer, producer, and director Sir John Boorman's house. His will states it will be given to Dame Helen Mirren.
When Morgana gives birth, Dame Helen Mirren's head is up through a hole in the table while a real pregnant woman lies on the table with her head covered.
According to Gabriel Byrne, his sex scene with Katrine Boorman was filmed with the two separated. He performed his close-ups in the scene with a pin cushion.