Unlike Dieterle , Boleslawski and Leo Mc Carey (directors of other Laughton films) , Von Sternberg wasn't prepared to follow his genius as he was not in for the 'thinking actor' approach.
He was used to have it his way or no way and was looking for a way out. Luckily for him and for Korda, whose company had already spent a zillion dollars on the project, Merle Oberon (who adored Laughton) had her car accident. What remains is proof of Laughton's genius and the greatness of the whole project. Derek Jacobi's portrayal, is equally masterful and both actors in my mind where examples of perfect casting.
Emlyn Williams comments on Laughton are sufficiently clear, praising his colleague and Merle Oberon who featured with Laughton in Korda's 'the private life of Henry VIII' was positive too.
Von Sternberg lost his touch around that time and never really recuperated turning him into an interesting but frustrated teacher at American film schools. One last thing : of course Laughton was not flawless himself and sometimes when he really put his teeth into a role he could be painstakingly meticulous and full of self-doubt even when he had mastered the whole character but the results were almost always worth the pain.
Ps.: It was Von Sternberg (still very frustrated) that didn't like Laughton 30 years on.