20 Things You Didn't Know About Star Trek III: The Search For Spock (1984)
5. You Klingon Bastard, You Made Me Lose My Footing
In the making-of documentary included with the Blu-ray release of Star Trek III, Leonard Nimoy explained how they approached the scene that immediately follows the death of David Marcus. Kirk, so sure that he can save the day, is crushed as his son is killed, and there is nothing he can do to stop it.
This is, with the possible exception of the mind meld scene, the one moment of the film that truly breaks Admiral Kirk. After Saavik reveals David's death, he steps backward, stumbling against the dais on which the captain's chair sits.
Nimoy gives full credit for the scene to William Shatner, clearly stating that he didn't give the man the direction to fall, but that the little stumble makes the scene far more powerful. Kirk has at this stage lost his closest friend, in the form of Spock, and is fairly certain that his career has ended as well. He hadn't counted on losing his son too.
It is one of the most powerful scenes in the film. Nimoy and Shatner rehearsed the moment on a closed set, allowing them to better prepare - and to attempt to fool the cast and crew with a huge, fake fight - one that Nimoy was certain fooled nobody.