Jim Broadbent won his Oscar from his first nomination for portraying John Bayley in Iris.
Jim Broadbent is not a supporting performance in this film as he is the male lead in late half of the relationship between John and Iris. Broadbent portrays the older John Bayley. Bayley as written is mostly a kindly old man, who loves his wife, and is almost complacent with his status as the husband of the more famous wife. This whole characterization Broadbent handles with a great deal of ease, and really it could have been quite easy to have messed up entirely.
Broadbent though is quite effective in his creation of Bayley. The various mannerisms such as his stutter, and his whole quiet shy demeanor are all very naturally portrayed by Broadbent despite the fact they all could have easily seem forced. Broadbent stays quietly honest throughout his performance which works for in his quiet supportive husband character. There is never hint of falseness to hid portrayal which is why his performance is effective.
Broadbent mostly stays loving and supportive of his wife. There is also a great deal of sadness over his wife Broadbent portrays quietly but truthfully. Also though there is some anger as well that he shows toward her due to the way he treated by her when they were younger. Broadbent realizes this as well showing it as rather deep rooted pain he feels from their history, not as just some sudden outburst.
The best part of his performance really is showing the way in which he slowly sees his position change in the relationship in life, and he begins to slowly gain a greater confidence. Broadbent carefully realizes this change in a slow but deliberate fashion. His best moment in this regard is probably his eulogy for one of Iris' friend, where although not a perfectly confidant man Broadbent shows Bayley having a greater grasp on his life. This is Broadbent strongest aspect of what I would say is a good performance, but for me it is the type performance I like, but do not love.