I rented an alien invasion movie in the mid-90s that I recall as being pretty good. I'd like to see it again, but I'm a little short on details. Really, I have only these:
The aliens impersonate humans.
A female scientist explains how some aliens she'd met felt wrong to her. When she meets men for the first time, she says, they invariably check out her body as they shake hands, however fleetingly. The aliens don't do it at all.
Some scientists examine a map on which the incidences of alien replacements/abductions/whatever have been overlaid. It looks like an shapeless blob at first, but someone realizes a river runs through the blob, and the shape can be explained by some alien presence moving downriver, spreading the infection (or whatever) outward at a constant speed as it goes.
At the time I saw the movie (mid 90s), I remember thinking that it had been made within the previous 5-10 years or so. On the basis of very little evidence I later guessed that it might have been been the 1978 remake of "Invasion of the Body Snatchers," despite being outside of the expected time range. However, I just finished watching that movie, and it definitely isn't the one.