How do I make heroes and npcs invisible?

It's easy to make heroes or npcs invisible. You simply give them a blank sprite (i.e. one that's all transparent) using the set hero picture or alter npc commands.

Examples:

#assuming that walkabout #42 is all blank

set hero picture(me,42) #make the first hero invisible

alter npc(2,NPCstat:picture,42) #make all copies of NPC 2 invisible

Remember: As with just about anything dealing with NPCs, changing one NPC's sprite will affect all copies' sprites.