His most celebrated film role was as Merlin the magician in the King Arthur epic "Excalibur" in 1981. Director John Boorman cast him opposite Helen Mirren as Morgana over the protests of both actors; the two had previously appeared together on stage in "Macbeth", with disastrous results, and they disliked each other intensely. It was Boorman's hope that the very real animosity that they had towards each other would generate more tension between them on screen. Williamson gained recognition from a much wider fanbase for his performance as Merlin. A review of "Excalibur" in "The Times" in 1981 states: "The actors are led by Williamson's witty, perceptive Merlin, missed every time he's off the screen." According to Mirren, she and Williamson, free from the problems with "Macbeth", "wound up becoming very good friends" during "Excalibur".