I also just rewatched it, and the subtlety of Robbins' performance also struck me. Dufresne is reserved by nature, and he's struggling to process his new life and the circumstances that led him to Shawshank. There are only a few moments when his stoic demeanor cracks, and they are all the more powerful.

Unfortunately, the Oscar rarely goes to the most subtle performance. More often, it is the most most performance that takes the trophy.