The invasion stories that disturb me the most are the silent, secret type. The big budget, let's destroy famous buildings and monuments approach is much less interesting. When I was young I was really creeped out by all the classic science fiction films that the BBC used to show. I vividly remember the horror of the idea of small town America being taken over by relentless Pod people who seemed just like us apart from a disturbing lack of emotion.
All sorts of different readings are possible for this film, and it has been linked to the politics of its era. Despite all this the writer, director and actors just thought they were making a good old scary movie. It's difficult to beat that image of Kevin McCarthy running though the traffic trying to warn his fellow travellers that "You're next!"
Invasion of the Body Snatchers is a great invasion movie.