The performance is a masterwork of restraint. Poitier does not push for your sympathy. He does not make Homer’s goodness explicit in the way that a lesser actor might, through expressions of warm feeling or speechifying about the right thing to do. Homer’s goodness is expressed through what he does — through the steady application of his skills to the problem at hand, through the patience he extends to people who are asking a great deal of him, through the singing.
The singing matters. Homer teaches the nuns to sing an English hymn — “Amen” — and the scene of that teaching, of these German women learning to follow his voice, is one of the most affecting in the film. It is also, quietly, a scene about who is the teacher and who is the student, about the specific knowledge that Homer Smith carries in his body that the well-educated nuns do not have and cannot acquire except from him.
The film did not have a large production budget, and Poitier’s decision to defer his salary for a share of the profits was a real financial risk. Films like this, without major studio backing, frequently did not find audiences. The fact that this one did — the fact that it earned enough at the box office and in critical recognition to place Poitier in the Oscar race — was not inevitable.
What was not an accident was the performance. The performance was the deliberate, sustained, decades-in-the-making product of everything Poitier had built, from the radio listening to the reading lessons to the ANT training to the ten years of film work that had refined his instincts and his technique.