I guess with me it's like a subconscious self learned habit. When I really don't have to be fluent I'm not. Peak stuttering for me is in a relaxed conversation with a small group, and it's slightly less when one on one with parents or good friends. But I'm most fluent in high pressure situations, like presentations, oral tests, interviews. Striking difference when working; I stutter often when chatting with colleagues, but almost no stutter when speaking with customers or suppliers.