You also need to take into consideration things like lighting: does the light in your video match up with the light on the grunt? Does the grunt accurately cast shadows on the environment in the video? Do things in the video cast shadows on the grunt? Also about shooting the video: is the focal length the same as it is in Amnesia (short answer: NO, very hard to get a 124 degree field of view in a real camera).
Above all else, is the camera actually going to move the way the view moves in Amnesia? VERY difficult to control, best to just matchmove the shot and import into a 3d app like blender, import the grunt and recreate the material as best you can, then insert THAT into your footage.
Check out this video of mine:
http://www.youtube.com/watch?v=f41ul3mjxSU