Never caught this before because the movie had been spoiled so long ago.
(Spoliers, obviously)
...but after Matthew meets with the man in the park, the director is clearly trying to give us an "is he or isn't he?" vibe about Matthew in the following apartment scene. Sutherland is unusually emotionless in this scene, and is pulling off some creepy stares. Also, we never find out what happened in that meeting in Union Square (apparently just a red herring both for Matthew the character, and us in the audience), which ramps up the confusion. Not until the pod attack do we know for sure.
Most of us have known the ending of the film for years, so we know that Matthew isn't changed (yet), but to a first time viewer these scenes would cast his identity in doubt, for a while. I'd noticed it done with Jeff Goldblum and Veronica Cartwright at other moments, but because I just KNEW Sutherland makes it to the penultimate scene, it never occurred to me to find him suspicious. Upon rewatching, though, he really is weird in this scene. Good misdirect by Kaufman!
The war is not meant to be won... it is meant to be continuous.