Herzog's first choice for the role of Aguirre was actor Klaus Kinski. The two had met many years earlier when the then-struggling young actor rented a room in Herzog’s family apartment, and Kinski’s often terrifying and deranged antics during the three months he lived there left a lasting impression on the director. Years later, Herzog remembered the volatile actor and knew that he was the only possible man who could play the mad Aguirre, and he sent Kinski a copy of the screenplay. "Between three and four in the morning, the phone rang," Herzog recalled. "It took me at least a couple of minutes before I realized that it was Kinski who was the source of this inarticulate screaming. And after an hour of this, it dawned on me that he found it the most fascinating screenplay and wanted to be Aguirre."
From the beginning of the production, Herzog and Kinski argued about the proper manner to portray Aguirre. Kinski wanted to play a "wild, ranting madman", but Herzog wanted something "quieter, more menacing". In order to get the performance he desired, before each shot Herzog would deliberately infuriate Kinski. After waiting for the hot-tempered actor's inevitable tantrum to "burn itself out", Herzog would then roll the camera.
On one occasion, irritated by the noise from a hut where cast and crew were playing cards, the explosive Kinski fired three gunshots at it, blowing the tip off one extra's finger. Subsequently, Kinski started leaving the jungle location (over Herzog's refusal to fire a sound assistant), only changing his mind after Herzog threatened to shoot first Kinski and then himself.
