Why was Brad Dourif's eyes watering?
was he having some sort allergic reaction?
shareLooks like the makeup dept. "painted" a reflective "tear streak" on Brad's cheek.
I think this was done to show the agony of the Gemini as a condemned being, and/or to show the "inner Karras's" suffering.
Applying a fake tear trail would be much easier than constantly coaxing real tears from Brad's eyes. So they probably gave him some kind of eye drops to make his eyes look "full", but not calling for real tears/active crying. Seeing Brad's full eyes, combined with the fake tear trail, lets the audience think that the fake trail is naturally flowing from Brad's eyes.
Just my guess.