The hardest part about playing blind...
...must be the eyes. I did occasionally notice Hepburn's gaze locking on to things, which bugged me, but it seemed to happen less and less as the film progressed. After pretty much looking straight into her eyes for the entire movie, I must say that I'm blown away by her performance.
I started wondering if she had some kind of lenses on, to actually simulate the blindness, but a few scenes seem to suggest otherwise. Granted, having never really spent time with a blind person, I don't know what the eyes actually tend to do, but I was convinced. This must be incredibly hard to pull off.