It doesn't quite work like that. There are MANY ways to make NPC objects. This is just one way, shown in the tutorials. Adding a "checkbox" doesn't do anything inherently. That would just allow us to write a bit. But what should that bit do? If the object became solid, it would then never be colliding. Then it could never see a collision, and could never register to show the NPC text. So that wouldn't work.
So one could, upon collision detection, do a check for a LARGER area than the collision data, but then you'd have to only use that data if it's an NPC type (otherwise monsters would register a collision using that data), which would be a LOT of wasted ROM space just to track that special case, and a lot of extra cycles, which would make the game slow down faster to do all those checks (welcome to NES).
You could include in collision with NPCs a routine that sets the player speed to 0 and player acceleration to 0, however you'd also have to set the npc's speed and acceleration to zero, and set them both to the previous position...but again, then you have that problem of them not actually colliding in order to enact the NPC dialog. So you could have a bit flip after collision for a handful of seconds that would be active and let the game know npc dialog is to happen, but then you'd have to also keep track of the last object you ran into, so you'd also need a variable for that.
You could use the object's melee - space to actually do the collision check for NPC rather than the object itself, since that space is already calculated and is tied to facing direction...essentially, when you *hit an NPC with a melee*, rather than hurting him, it does the NPC, and then you can use the above solution to make them solid...you'd also have to probably set it so an NPC changed direction during that step so that they don't get *stuck together*.
These are just some ideas. There are literally a thousand others. Just like there are a thousand other ideas for the HUD, or movement scripts, or collision scripts, or powerups, or weapon usage...etc etc etc. Game by game, the solutions and best way to handle these things will be different. These modules are just a starting point, and intentionally so. As time goes on, more and more ideas will be exposed on how to do things, by us, by other users, here on the forums...
If you guys are 1 day into NES development, and already have working NPCs that give you selected messages...just understand you've just gone through lightspeed to get to this point. Haha