Sorry, I missed this earlier.
What I do and what Milos probably means is what I call "NPC walls". So, instead of using a sprite which can only appear in the framework of events and will disappear afterwards, I take the NPC and put him in different sizes in the respective slots of the wall template. In most cases, I only show the front of the NPC, assuming that he/she/it would sense the party and automatically turn to the party members. But in some cases I made two or even four different views of the same NPC (behind, front, sometimes also left, right). A nice example is "Friedrich's Revenge", where you can sneak into a building while a monster shows its back, but only at certain times. If you pass by at the false time, the creature will see you (thus show its front).
With this approach, you can quite easily do what Milos wants to achieve, including stealing or attacking from behind, and you can do it in different ways. And in case you want to move the NPC, you can use the options to change walls (script/logic block).
The concept in most of my designs is that NPCs who generally stay at the same place (guards, barkeepers, traders etc.) are depicted as "NPC walls", while characters who wander around are handled with sprites.