How do I tell AGS using the interaction editor to animate a hotspot once The player character has done a certain interaction with it?
Could you please explain what you want to do? Hotspots don't animate - they can however trigger animation in a character or an object. If you need to change a part of the screen when it's clicked on (to open a door etc.) you'll be better off using an object.
Better of using a door as an object?
Oh... So there is no way I can interact with a hotspot in a certain way and therefore change the background to a background where the door is open slightly more and then more and more till it's fully open? That would need to be an object fit onto a background where the door should be?
Well, IN THEORY, you could use background animation, if you only had the one thing in the room that you wanted to animate (See SetBackgroundFrame(..) (http://www.adventuregamestudio.co.uk/manual/SetBackgroundFrame.htm)), but using objects is probably the better way, yes. It gives a little more control, more frames for a smoother animation, and allows more than one animated ting (doors, windows, TV screens) per room.