They will sporadically spit a red bullet into Isaac's direction until the room is cleared from regular enemies, although they will sometimes attack 1 extra time after all the enemies are defeated.
Although indestructible, tear effects will still affect them, so it is possible to decrease their shot speed with Spider's Bite, or their range with items like Little Gish. It is also possible to stun them briefly with Mom's Contact.
They are considered regular monsters by blue flies or familiars like the Dead Bird, which might end up attacking one even though this will have no effect.
In Wrath of the Lamb appears an alternate version of the regular stone grimace. It is tinted green and spits green bombs like Sloth that will create an explosion and cause poison damage upon impact. They are commonly encountered in both the Depths and Necropolis, occasionally even in Utero. The bombs they fire can be used to your advantage, as they will not only harm you but your enemies as well.