my company puts real people into digital sets all the time. sometimes it looks better than others and I think i have narrowed down why.
1..Angles of the feet – they need to somewhat match your scene…this is probably obvious to most..but not to our video department. If you shoot at a high angle make sure your scene is set up as such.
2. placement and rotation. there is a script we use so make sure the actor is always facing the camera so no matter where the camera goes, the actor never distorts..Also make sure your actor is where they need to be by using all 4 views..make sure those feet are on the ground or close..
3.. shadows and reflections…without a shadow it looks like they are floating. and shadows are hard..try using a real light to cast shadows on a place..or vc reflect is pretty good…you just might have to animate the position