Good, using this boolean you can pick which NPC is currently talking or interacted with (system event "Pick by comparison"), and then create the HUD sprite next to the NPC, and in the same action set the HUD frame/animation based on the NPC variable, for example if variable = Happy, HUD : set animation to NPC.(your variable), and when exiting the dialog, destroy the HUD object
Tomycase
I never used this "Pick by comparision" function before... :(
If it's not asking too much and/or take your time, you can make an example to me that working?