Some add-ons as someone who has stuttered since childhood:
A stutter can be accompanied by a sudden change in volume.
It can happen at any speed of speaking.
Mentally, it can feel like a quick stumble, a jolt of electricity, slamming into a brick wall, or reaching for a ladder rung that doesn't exist. Or any other number of metaphors.
The person stuttering may be surprised, or may have seen it coming.
The rate of stuttering varies greatly from person to person. There are extremes in both directions. And even one person will have good days and bad days.
A person who stutters may be shy, or they may be an extrovert. Possibly even a confident public speaker. (me)
Pausing and/or switching words on the fly may be a tactic a stutterer uses to feign fluency.
Other coping mechanisms may non-verbal, like pointing to a menu item when in a restaurant.
A stutter can be accompanied by a physical tic, like wincing or stamping a foot. The tic might be self-inflicted as a way to ease other people's discomfort with the one stammering. Some stutterers used to have a tic and have drilled it out of themselves (me).
Some stutterers appreciate having their sentence finished. Others despise it. It could also depend on who is doing the finishing.
A stutterer may constantly fight the perception they are shy, excitable, unconfident, etc.
A stutterer may use their stutter to their advantage if they want to be underestimated.
People respond to stuttering in a variety of ways - I've experienced everything from being mockingly parroted to people denying I even have a speech impediment.
Stuttering can sometimes be tied to fatigue, exictability, or context switching (happens to me a lot when shifting languages). But not always.
Some stutterers have done so since childhood. Some may have gone through speech therapy. Others acquired the impediment via some kind of brain trauma.
Other common times when someone may *not* stutter: taking on an accent, acting, talking to themselves, talking to a pet.