Well i have a theory which I think is pretty accurate. I have seen guys like Bruce Willis and Rowan Atkinson do some interviews and being a stutter you can see when they are about to stutter or were about to stutter because of the way they speak or how their mouth contours but then they make it fluent. Just because they don't stutter in front of camera doesn't mean they don't stutter. Steve Harvey also has a stutter and he once had a person on his show speak up and she was stuttering while asking question and he then gave someone's number to her to improve her stuttering. There is another great bollywood actor called Hrithik Roshan , he stuttered alot and says he still does and the only way he doesn't stutter as much in public at least because of his speech therapy and practice. He once practiced one line for an hour or two just to accept an award. You just see the results , you don't see the practice or the work behind it. I saw his interview and you can see he still has a stutter because he elongates the first alphabet of some words and he does stutter in some words but because his speech was fluent the whole interview it did not even feel like he stuttered. Felt like he just repeated the alphabet couple times really fast. He also says that if he doesn't practice on his speech he stuttered alot more. So, we just see the 1% of their speech and think of they don't stutter and they are just mild whereas we don't know what is going on behind the scenes.