How to make NPC's look different when based on Fuse character or other character model

Hi, I want to have human NPC’s and was wondering how to go about taking a couple of Fuse characters and then modify them at run time so they don’t look like clones. So in broad strokes how is this achieved in games? I am not looking for a plugin, more of a tutorial or explanation of what methods are used so I know what I need to learn.