Actually, there is a problem with the solution.
Putting the placeables in an inaccessible area wont work.
The Placeable has to be within range of the player, to be involved with the conversation.
The Placeable will be acting like an npc, so the Player has to be able to turn his head towards it etc.
I've found that unless the placeable is nearby, within 1 tile of the player/creature, then the conversation will fail to fire, or will cease when it gets to the node where the placeable is due to talk.
Resolution for me was - Simply make the placeable invisible, and paste it close to where you know the Player is going to be.
If this is going to be dynamic, called from a script, where the player can potentially be anywhere in the module, then simple solution is, Create the Placeable, at the Players Location, and make sure its set to be an invisible placeable.
That way, it is summoned to the Player, you get the portrait, and people will be none the wiser. However, there is a slight aura glow from the Invisible placeables from CEP23.
Check my video where I use the same system.
When the player - Me, gets outside of the Building, a conversation ensues, you can see for an instant, the source of the npc voice is an invisible placeable on the ground, but thankfully, it only shows for an instant.