For those of us who stutter, see the film again. Firth really learned the nuances, and not just the facial and verbal features of a PWS. Before and even part way through ‘therapy’, you see very little eye contact when he is talking to others, with the possible exception of his wife and children.
This improved as the film progressed; just as we get more confident and more positive about ourselves, so did he. Other aspects, such as posture when sitting, standing; sitting he appeared to be in avery enclosed ‘space’ – arms close to his body; slightly stooped when standing (looking down). This also improved as his ‘therapy’ continued. As with us, it is not a gradual linear relationship, but one filled with peaks and valleys.
See it again, watch closely, and you will really see ourselves.
I can see why Colin Firth won the Golden Globe award for best actor because, as you say he has so accurately captureed the nuances of the body language of someone who stutters. Just one reason among many others to see Firth’s incredible performance.