Note: Imagined words may be deceiving in their appearance. They aren't performed, so you don't know if they would actually manifest in the desired fashion. Or in short, speech doesn't amount to "form sentences, deliver through mouth". Sure, you can e.g. write something down and read it out loud, but it still doesn't mean performance is determined. You're not doing it in advance, after all (rehersal is not premiere, so to say). I think the reason we stutterers note a contrast more than most is that there's often far between what we imagine and what we perform. But it's a generic thing. In other words: "Thinking out loud" would not necessarily make you fluent and charismatic.