In the original film, Miles kissing his girlfriend, and she's one of THEM.
There's a particularly chilling event in the background towards the end of the 1978 remake - we see a group of (unconverted) children on a field trip being taken into a building. You can hear one of them complaining about how they don't want nap time already.
Late in the 1978 version, where the original version of Elizabeth disintegrates in Matthew's arms, and the duplicate version appears nearby. The 1993 version has similar effects for most of the main victims, and while they don't quite have the creep-out factor of the preceding version, they're still pretty discomforting to watch.