Looking at science fiction and horror film subjects, it struck me that more than 'The Invasion of The Body Snatcher's,' focus on fear of identity loss or not fitting in – in fact this is the main subject of various film genres: The Thing (John Carpenter's version) and Blade Runner come under this heading as does Alien by Ridley Scott. Vampires, werewolves, zombie movies, not to mention films about possession, cover this area; medical dramas too (fear of contagion).
Even class and snobbery (nouveau riche / U and Non-U), crime (imposters, thieves, counterfeiters, fraudsters etc) and politics (faking sincerity) are covered by this heading as is espionage (infiltration, sabotage, betrayal, coded information) and behind that the fear of different beliefs (communism, Islam to name but two instances). Star Wars is about attacking the status quo and Star Trek is about defending it (the law / stability). In TV land we have the Daleks and their dislike of the unlike, plus Cybermen and The Borg, who want to assimilate the different.
Youth and age, science and superstition, Frankenstein's monster and even followers of fashion (wannabes) carry on this battle to conform or rebel against conformity (become unpredictable, through either strange behaviour, new ideas or difference in appearance). Everywhere we look this combat rules our every move or lack of it – to join or not to join, that is the question?
Think of how develop as human beings. We start off as a clump of cells that then turn into a foetus, which like Ridley Scott's Alien, mutates again and again, going through several forms until it turns into a recognizable baby and gets born. After birth it continues to grow and change through childhood until it hits puberty and a new volcanic eruption occurs as it becomes mature enough to start the cycle of another being itself.
Beyond this it develops until it loses its usefulness as an individual and starts to fall apart as in a horror movie (death, the biggest change of all, after birth and becoming). We struggle to get established as an individual or group, then fear losing losing this position in time and space but do so anyway as life isn't stagnant (the only constant in the universe is change, yet we fear it because it alters who we believe ourselves to be – hence identity crisis as a threat to this (death of the old, birth of the new and different – the chrysalis effect).