I’d like to second this. If an actor wants to change a line so that it creates a more natural delivery, I encourage it, but you need to take notice of HOW it is being changed. On the last feature I did we had an actor who would constantly change his lines. The problem we had was not that he was changing his lines, but he was changing the meaning, sometimes changing it to the point of the line he was delivering having the complete opposite meaning to what he was meant to be saying. We were fortunate enough to have someone on location supervising the script and calling out whenever this occurred. So just be mindful of that.I second all the replies here. I am currently writing a script. I have written several but I use the script as a guideline only on set. I would rather speak to the actors beforehand and get there feedback first. Some actors actual will read the script word for word and that is their personal work ethic. But that could lead to a wooden performance. If an actor has any conviction in the part they will contribute to the script which is far better as they have an emotional attachment to the character. I hope this helps.