I've got another problem now...
I have two non-player characters characters in a room with my player character. I walked my player character up to one of them, and he's got stuck. Looking at the walkable area with Ctrl+A, I see that the NPCs have small black rectangles around their feet - presumably this is to prevent the player character from walking into them. Is it possible that somehow I have walked on to one of these rectangles? My NPCs do not move yet, by the way (which is correct for the time being).
Thanks.
It depends on what the walkable area round the NPC looks like (i.e. if there's another way for the PC to get past them), but it's possible. Try disabling their Solid property (http://www.adventuregamestudio.co.uk/manual/Character.Solid.htm) (can also be done on the Characters pane of the editor) and see if that does anything. If so, you can either leave them as totally not solid, or adjust their BlockingHeight/Width properties to reduce the area they block.