...but an office worker has some sort of perceptual shift, and stops following his pattern--wakes up.
Other people keep speaking as though he's replying with the preset line, but he's just listening in horrified silence. They also move in preset patterns--disrupting their movement makes them have seizures.
Eventually the protagonist meets others who've woken up: a woman whom he likes, as well as a gang of thugs (who have a large/evil hound) who run around pillaging and (I think it's implied) raping the unknowing "machine part" people.
At the end the protagonist and his lady friend decide to rejoin their patterns, by relaxing, dropping back into their routines.
I think it's from the 50s/60s, but I can't for the life of me remember who wrote it, or the title. I think I might've run across it in an anthology of short stories.
Any help is greatly appreciated!