In base game zones and I older DLC zones using the interact key on NPCs that don't have the 'talk' button appear causes them to look at and acknowledge you in some way, usually in the form of a greeting.
It appears that this function has been removed in Markarth, with every NPC stonewalling you wherever you go. That is, they don't react when you use the interact key on them. No acknowledgement or even a head motion to look at you. I always appreciated this minor acknowledgement from NPCs, and it made the game more immersive, as most NPCs had at least something small to say. In Markarth, the lack of this feature makes walking through the zone, only able to talk to NPCs with dialogue, feel less engaging, and the world is less alive.
It's these small details that really enhance gameplay, and its a shame that this has been overlooked in this DLC. Hopefully, this removal is reconsidered in future content.