R273 R274 - I'll offer a middle of the road take on that excellent Julius Caesar:
I don't think Brando was awful, in fact, considering the cast he was part of, I was surprised at how well he did.
That said, I do think the gap between him and Gielgud, Mason, and come to it, Calherne (Caesar) was evident, and comes down to tradition and/or training. Mason wasn't formally trained, but did a great deal of stock theatre before becoming so well known for film work. The whole tradition of navigating the crossover between literature and performance in the UK is still very strong, although I'm not sure how long it will hold up in the post-modern era, when so much is visual and audible, and the printed word has lost so much value.
Calhern was also primarily a stage actor for quite awhile before returning to film. You learn a different craft on stage than on film, and when the two genres are blended in filmed Shakespeare, interesting things can happen, and the gaps in one or another's skills get highlighted.
Brando, however, left the live stage very early soon after his early success in "Streetcar".
When Brando had to put it over a crowd he had to scream. The others were either taught or learned in years in theatre real voice projection. That was one difference.
For all Brando's star power, for me, the relationship between Brutus and Cassius, served up by experienced hands, carried the film
For what it's worth, here are Brando's comments on American realist theater and film (Stanislavski), and Shakespeare:
"This school of acting served the American theater and motion pictures well, but it was restricting. The American theater has never been able to present Shakespeare or classical drama of any kind satisfactorily. We simply do not have the style, the regard for the language or the cultural disposition ... You cannot mumble in Shakespeare. You cannot improvise, and you are required to adhere strictly to the text. The English theater has a sense of language that we do not recognize ... In the United States the English language has developed almost into a patois."