Peter Finch's character here is quite original, and unique for its time. This because his character is actually fairly comfortable with his homosexuality, although still hides it from his family. The fact that he is an homosexual is not shown to be Hirsh's trouble in his life. His problems rather come from his lack of love in his life, and his concern over the soon departure of his lover.
Finch's performance actually was a little surprising for me in that he is far more withdrawn than I expected him to be. It is basically the polar opposite of his winning performance in Network. Finch's performance here is especially quiet throughout the film. Finch's performance much of what he shows is a rather simple facade of Hirsh's he presents to his patients and his family.
Finch handles this quite well because he does not show it to be as putting up a front of a completely different person than himself. It is instead in many ways a repressed facade of himself he uses to get through his day to day life. He presents the doctor to his family and patients as a quietly charming rather seemingly simple man who seems content enough with his life.
Underneath that Finch puts quietly though a sadness that is perpetual in the character. It is quite subtle but well handled by Finch, as he shows behind his somewhat content facade is a rather sad man, who clearly yearns for love. I think this is especially well shown in just the opening scene where he is talking to a patient in a kindly doctor fashion, but when answering the phone he shows his sadness over his life in a brilliant subtle fashion by Finch.
Hirsh though does show even more himself, although he still hides a quite a bit of his constant sadness due his knowledge of Bob's eventual departure, when he is with his lover. Finch is especially good here because he shows a difference between his actual happiness he has with Bob, in contrast to his false contentment he tries to show otherwise. Even here though Finch does still show the undercurrent of sadness to his character.
A subtle strong performance throughout the film, and I find he is particularly good in his one scene with Glenda Jackson where the two different yet very similar characters have a meeting. It is a great scene where they meet with one another. An there is a understanding between the two that is exceptionally well shown between the two actors. I think Finch gives a strong performance, although I never became overly invested into the performance.