This was the second time that 'O'Toole, Peter' 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 (I)' as "pig" during filming. Every day at five o'clock the two would unwind over a cigarette and a glass of white wine.