Excalibur (1981)
Nicholas Clay plays Lancelot. What makes the film unusual is that Boorman shoots the legend as the legend instead of as a historical drama.
Actor: Nicol Williamson
Nicol Williamson played Merlin in Excalibur, a performance of complete strangeness that the film builds around, and John Boorman cast him opposite Helen Mirren knowing the two disliked each other, which he used. He was regarded as one of the finest stage actors of his generation and was famously difficult.