It's best to ask them about it, they know best what helps them, what they feel comfortable with, etc. Apart from this, just just interact with them like you would with a non-stutterer. You could watch some documentaries about stuttering, stutterers, see my [comment ](https://www.reddit.com/r/Stutter/s/tGpUKjgMRM) about some documentaries. They can give you a lot of insight into stuttering, how stutterers feel, what they go through.