It's not easy being in every scene and acting against an off-camera voice. I think Samantha Morton was giving him good material to work with and they had the luxury of making a lot of dialogue changes in post when Scar Jo came in, but it really was a powerful and understated performance. Best I've seen from an actor since John Hawkes in Winter's Bone.
Her crept over my head like a cowl and kept me enthralled for 90 minutes. Not a lot of movies do that for me.