This trophy is easily missable as many character models only appear in certain missions, I recommend following this guide from the very start of the game.
I personally missed it the first time and had to put an extra 3 hours in going through most missions again to finally get to the 40 needed.
These are 40 different chracter models, not simply turn into people 40 times.
Credit to the YouTube user Nano4211
Feel free to comment any suggestions