Original engine bug.
Any controllable NPC with cycling animation (elementals, hexed followers) will stop if you check his character sheet.
Elementals will also stop emitting sound. They will remain in this state until there is a need to move.
It is much harder to make several animated NPCs to freeze at the same time. One will 'wake' when other freezes, or they 'wake up' quickly on their own.
Save-elemental.zip