How do I talk with a person who stutters sometimes?

How do I talk with a person who stutters sometimes? I work with a bloke who has a bit of a stutter. It's not like it's debilitating or anything, but it's the first thing I noticed when I met the dude. Old mate's living his best life, he definitely doesn't need my sympathy. I just thought it might be worth throwing the question out there. How do you talk to a stutterer to make the interaction a bit smoother?