Normal demons are like vampires where they can only roam in the night because the sunlight burns them to a crisp.
On the rare occasion that Nezuko attempts speech, she has been seen to stutter a lot, which may be due to her bamboo mouthpiece, which is seldom removed, and the fact that she didn't speak for a number of years after her transformation.
He tilts his head and crosses his arms at Nezuko, and she imitates him in return.