Joshua will not need to see any skills used in order to mimic them, or even know what the skills are, but he will need to be near the person he is mimicking from. If he then targets that person by thinking of him or her, all of his or her skills will be mimicked. He will never need to practice his copied skills, and they will not fade no matter how much time has passed since they were originally gained, or how many other skill sets are copied. Joshua will be able to access any skills at any time after they have been mimicked.