This was the second time that Peter O'Toole played King Henry II. The first time was in Becket (1964). He received Academy Award nominations for both performances.
Katharine Hepburn is descended from Elinor of Aquitaine in numerous lines, from both Elinor's marriage to Louis VII, King of France, and Elinor's marriage to Henry II, King of England.
While filming close-ups of Richard (Anthony Hopkins) in his jousting costume, the horse was spooked and bolted. Hopkins fell off and broke his arm. Filming the scene with his sword raised above his jousting opponent was very difficult due to this.
Although Peter O'Toole plays the father of Anthony Hopkins, John Castle and Nigel Terry, he is only five, seven and thirteen years older than them respectively. Moreover, O'Toole is twenty-five years Katharine Hepburn's junior but plays her husband. It should be noted, however, that there was quite a substantial age gap between Henry II and Eleanor of Aquitaine - she was approximately eleven years his senior.
Eleanor greets King Philip by telling him that she could have been his mother. This is in fact, true. Eleanor's first husband, Louis VII, later fathered Alais by his second wife and Philip by his third. Eleanor's marriage to Louis was annulled by the Pope when she was unable to bear him sons. She gave Henry five boys, two of whom died before the action of the movie takes place.
Katharine Hepburn affectionately referred to Peter O'Toole as "pig" during filming. Every day at five o'clock the two would unwind over a cigarette and a glass of white wine.
The original Broadway stage production written by James Goldman opened at the Ambassador Theatre in New York on March 3, 1966 and ran for 92 performances. The cast included Rosemary Harris as Eleanor of Aquitaine, Robert Preston as Henry II, and Christopher Walken as King Philip of France. Rosemary Harris won the 1966 Tony Award (New York City) for Actress in a Drama. A 1999 revival starred Stockard Channing as Eleanor and Laurence Fishburne as Henry II.
According to Anthony Harvey, the director of The Lion in Winter (1968), Katharine Hepburn kept the Oscar she received for the film in a paper bag and in a cupboard for years after he'd delivered it to her.