A few blood stains and bullet holes never stopped you from making a proper disguise. When critters with outfits die near you, you will often be able to salvage a piece of their outfit for use with a disguise kit.
E.g.: Ted has his own Blackspear Tribal disguise, but he wants to take his CNPC pal, Destructo, into the bunker as well. A random encounter finds him watching a group of Blackspears fighting raiders. Ted and Destructo sit the combat out until the Blackspears get annihilated. Ted then walks up and gingerly searches the bodies, finding a Blackspear headdress, gooey black facepaint made of... something, and one of their patchwork brahmin-skin outfits.
Behind the scenes
The perk gets its name from Invasion of the Body Snatchers in which aliens took the appearance of humans to deceive other humans.
The perk description was posted by J.E. Sawyer on the Interplay forum in 2003.