Stand-ins are just the same height, and colouring possibly. to do all the boring stuff like getting the lighting right on each scene without our boy having the tedium of standing whilst the setting up is being done around him, and everything is measured and marked so the stand-in can be used to work out exactly where every mark needs to be to get the shot right. There are all sorts of other issues to take into account like eyeline matches and the CGI markers all of which the stand-in can do then Sean just wanders on to the set, stands in the right place, does his magical thing and voila!
My mother was a stand-in on a film solely because she was the same height, build and colouring as Lauren Bacall and the only woman in India at the time who was so ... she got to stand around in astonishing heat under lights whilst people measured everything, rehearsed and planned explosions around her, and walked through a railway carriage full of 'dead' extras who were touching her legs as she did as they had not experienced a white woman's skin close up like that. Dull stuff for a star, especially in that heat, but a coup for my Mum and very well paid!
