I noticed that if I have an actor firing bullets with a wait, if I repeatedly press start the bullets are fired continuously without pause. I cannot solve this in any way. I have noticed the same problem in many games made with gb studio.
Here's another idea. This might be the best one yet.
The current GB Studio version has a "Script Lock" event that pauses everything except projectile motion and sprite animation. If you place that before your dialogue event, you won't have to worry about bullets firing too soon!