James Mason did not receive an Oscar nomination for portraying Brutus in Julius Caesar.
Although Marlon Brando was Oscar nominated for lead for the film is was much more of supporting in the role of Marc Antony although he certainly makes a strong impact with his relatively limited screen time. Mason plays the true lead of the story of Brutus who is one of the chief conspirators against Caesar, but it is notable that he also is Caesar's friend. The first act of the story focuses on Brutus as he decides and eventually prepares for the assassination of Caesar who has been seen as having become tyrannical. Mason, despite technically playing a conspirator in a coup, portrays Brutus in a fairly straight forward fashion. This might seem an inconsistency but Mason technically takes the right approach for his Brutus. Brutus is the only man with power who actually does what he believes is best for Rome unlike all of his other compatriots and even Marc Antony who show far darker shade of themselves after the assassination takes place.
Mason takes that approach and is quite good in taking this approach. Although Brutus's speeches come later in the film Brutus is fairly reactionary at first as he listens to the other conspirators who are trying to convince him to come to their way of thinking. Mason even when not having the showy performance at a particular moment knows how to carefully makes his impact in these scenes. Mason is excellent in every reaction of Brutus as the time moves closer to the assassination and more men reveal their plans. Mason conveys conflict brilliantly in his expression as he portrays Brutus as seemingly always calculating every element involved with the plot while still carrying a certain emotional volatility reinforcing the idea of how dear Caesar was towards Brutus. Mason expresses the differing thoughts in Brutus of the situation quite effectively as he portrays the pressure in him as he seems to be trying to find what is truly the right thing to do.
Where the other conspirators are all portrayed with a certain level of deviousness Mason appropriately shows not the slightest hint of it in his portrayal of Brutus, and we all know Mason's a man who knows how to be devious. When Brutus decides to take part in the assassination of Caesar Mason does not portray Brutus as a man seeking power or in some way lusting for violence. No instead Mason carries himself with a very specific passion in these scenes. In Mason's eyes he presents no evil rather a drive to do what he believes is necessary to prevent Caesar from destroying Rome through his growing tyrannical attitude. Mason makes Brutus a truly righteous man in his manner and there is never a doubt that Brutus is the most virtuous man in the film. Mason earns his place by making Brutus the honorable man he should be by showing that in every step of the assassination that Brutus understands the gravity of it.
The assassination scene itself is fantastic scene for Mason as the men go over one more issue with Caesar as the men all take their positions to murder the man. Mason is great in the moment as Brutus asks Caesar to reconsider the issue as Mason shows the dire need in Brutus to try to reach Caesar even if one can see the same futility in his expression as the men approach from behind Caesar to end his life. Mason is particularly effective in the important moment where Caesar literally asks him if Brutus as well is part of the betrayal. Brutus rather bluntly confirms this by stabbing Caesar once more. Mason does not portray this action as the same way the other seem to stab with a certain maliciousness. Mason instead continues to portray the conflict in Brutus even in this moment as Mason expresses still a dismay at betraying one he loved and respected even while he must fulfill his own perceived duty by killing the man.
After the assassination Brutus must attempt to explain his actions to a volatile crowd and it is interesting to compare Mason's performance from Brando's speech that comes right afterwards. Mason's portrayal is far less intense than Brando's but this actually makes sense because Brutus is trying to calm the crowd not rouse them in anger. Mason in this way is equally effective in portraying Brutus's equally impassioned yet far calmer speech to the crowd. Mason keeps his emotions more focused on point as he shows Brutus's speech to be one of more of a politician pleading for understanding opposed to the hellfire preached soon afterwards by Antony. Mason carries himself with the necessary command in the scene and makes it believable that Brutus would be able to calm the crowd for the moment. It's a great scene and technically slightly thankless in that he does technically allow Brando to showboat, but Mason's humble approach is the approach that works best for the story.
Antony's speech forces Brutus and his allies into a war in which it seem they will likely lose. Mason still keeps Brutus as a righteous man in his steadfast approach to even confronting his closest allies if their is corruption in the ranks. Mason in this later scenes though is very moving in portraying a slowly growing grief in Brutus as he sees all his dreams fade to nothing in his losing crusade and things are only made worse by learning that his wife has died. Mason does not necessarily portray Brutus as being filled with regret or remorse, but rather as a man being forced toward a doom. Mason shows Brutus as a man who's true passion, the preservation of the Roman republic, to ever wain although he brilliantly undercuts in reflecting the way he is being torn apart by not only the killing of Caesar, but all the other tragedies that have come to him. Mason is heartbreaking as he makes Brutus's final fate a natural progression of a man who technically never lost his way.