Sam Neill did not receive an Oscar nomination, although he did receive an AFI nomination, but since Reb Brown received one of those one can perhaps question their validity, for portraying Alistair Stewart in The Piano.
Sam Neill portrays the plantation owner, and really this is a role one can probably predict quite a bit about from the moment you see him on screen. The other man in the love triangle is a role that can easily be an extremely one dimensional character. These characters sometimes can be just extremely dull straw men simply there to show how great the other romantic lead is, or they can be just one dimensional evil villains who are evil just for evil's sake it seems. Although I won't say that the part of Alistair is that complex of a character, he thankfully is not either of the two types of the other man used in lesser films.
Alistair is a character who has the tenants of these sorts of characters is that he is the richer man, he is the first man, and he really does not understand Ada very well. Well Neill should get a great deal of credit here in that he does not settle allowing Alistair to be a simplistic character in that he does a very good job in showing that Alistair's reaction to her is not something because he is too little of a character to understand, but rather he genuinely does not know what to do with her. Neill actually conveys an earnestness in his character that portrays Alistair as a man who honestly wants to understand but he can't.
Alistair is not a perfect man in any way, far far from it in fact. Neill though plays him as a man who is attempting to be some sort of husband, and father but really can't quite fit the role. One particular good moment early on in the film is when he trying to comb his hair properly as he is going to meet Ada for the first time. Neill does well in showing Alistair's attempt to turn himself into what he believes he is suppose to be, but really there is that lack of experience and fear of sorts underlying in his face. Neill creates this problem well in Alistair that works quite well later on in the film.
Later in the film when Alistair finds of the affair between Ada and Baines, Neill is quite good because he does not portray it as you might expect. Again Neill is good because he does not portray simply anger in Alistair, as he really has not become that attached to Ada, but rather he quite effective making the reaction one of bewilderment. Alistair is not at all relieved by her continuing going to Baines even after saying he knows, and later even tempting Alistair, but not allowing him to touch her. Alastair's reaction to the affair soon becomes extreme, than later becomes quite reasonable, this would seem hard to believe but Neill makes it work.
Neill makes the two actions of Alistair first one of extreme violence toward Ada, than later one of understanding. This works because Neill's depiction throughout showing Alistair as a man trying comprehend Ada. Her actions that throw him into violence is shown by Neill as the worst possible thing for Alistair, as her continued actions against his will only lead him to a worse state of inability to discern her. Neill makes his violence towards her less of actual hate, but rather of frustration. His just as extreme turnaround is actually earned by Neill because he portrays a realization in Alistair. This works because it is not of Alistair finally knows what Ada wants, but rather simply that he is better off without her because he has not idea how to understand her. Neill does exactly what he needs to in this role, making his character's action entirely believable. With all of that said though I would say while good in the role he is not what your remember from the film. Both Harvey Keitel, and Neill are overshadowed by Holly Hunter's performance. Neill is most certainly good, I must stress that but his performance simply is not where the power of the film comes from.