Adaptation Displacement: The number of people who've seen the movies dwarf the number who've read the book quite handily.
Alternative Character Interpretation: There's some room for uncertainty regarding whether the aliens are truly replacing the originals, and taking their memories is merely useful for blending in, or if they actually see themselves as merging with the original individual in a symbiotic relationship. This can make the aliens either a case of Always Chaotic Evil, or simply Blue and Orange Morality.
Paranoia Fuel: Have your friends and loved ones been acting strangely lately? How strangely? By the way, if you fall asleep you might be replaced by an alien doppelganger. FYI.
Uncanny Valley: In universe as well, at least in the original. The duplicates look like, sound like, act like and have all the memories of the original, but their close relatives know something is wrong.