Equip the Altair costume after unlocking it. There are some changes Snake will make to his posture. First, he stands with his hands by his sides and outstretched just like Altair, as opposed to hanging down in front of him with his fists clenched. This change will only occur when you do not have a weapon equipped. Second, where Altair had the ring finger on his left hand missing, Snake bends his up to give a similar visual effect. He will even keep his finger bent during intermission sequences in which the costume you are currently wearing appears.