Wall Eyes are only encountered in the chapel room of the Versailles Palace stage. While both John and Eric encounter them during their respective journeys, they play a more prominent role in the Eric's, as he'll be forced to climb up the entire chapel room since he has no means of crossing the large chasm on John's route.
They are encountered in big numbers attached to the walls and ceilings of the room, where they slightly move back and forth. When one makes direct eye-contact with the player (that is, when the player approaches the straight line of sight of the eye), it will briefly stop and then shoot a laser beam straightforward. If the eye is too far away from the player, it will just close its lids and remain passive, even if the player crosses its line of sight.
Other than that, they're not hard to deal with, as they can be killed in just one hit with most weapons. Their main threat is the sometimes hard to reach spots they're located on, which may put the player at risk while trying to hit them.