David Niven is one of those actors that seems like he must have been nominated for an Oscar more than once, but no, he was only ever nominated for this film alone. Luckily it was a deserving nomination, although I would say it certainly has supporting screen time, but his role does have the importance of a leading role.
Niven is superb in every scene he is in even if he is only in a few scenes. He carries himself perfectly in this film. He gives an interesting performance because he does these various mannerisms, in his voice and his posture that one would expect from a British officer, except they seem slightly false. But the interesting part is they are suppose to be false, and forced really. Niven does this false routine just right, so it is not obvious but is at the same time clearly off in some way.
David Niven stays very restrained with his short performance but creates a very powerful performance. Niven focuses mostly on his face, which Niven uses to great effect. He shows a history of the character with his face. Some of his expressions are especially effective such as when he looks at the newspaper that tells of his act, or when he find out that the others found out. His scenes with Deborah Kerr are brilliant and both play off each other very well, well their doing these incredibly introverted characters. Niven is really perfect here showing the shy nature of the character impeccably well, along with his attempts to be a "normal" soldier. Niven is never off and makes the best use of all he has giving a very good performance.