To start a conversation, multiple scripts are available from the core game resources. To have a placeable start a conversation when it is used, set the placeable's conversation then set its OnUsed event handler to nw_g0_convplac. The conversation is set under the "Advanced" tab in the placeable's properties. To have a door start a conversation when it fails to open, set the door's conversation then set its OnFailToOpen event handler to nw_g0_convdoor. The conversation is set under the "Advanced" tab in the door's properties.