Right, and to elaborate (I've only worked on a few major studio films in the late 90's, but I'm sure it's still the same): * Major projects aren't given the go-ahead until an A-list actor is attached, and usually they don't want to pay someone to write the full script until then (they just have a basic treatment and outline). Shooting though revolves around when the actor is available, so there can be a scramble to get it finished, and they're still polishing it up until and after shooting has begun.

• Pre-production they'll delete or modify scenes if it looks like there will be cost/schedule/logistical issues (and will ask the writer to do that)

• While shooting, they may run behind schedule and need to cut or shorten scenes yet to be shot.

• Some minor character is working really well and they'll ask for more dialog/involvement for him

• Post production they'll decide to modify or add new scenes or dialogue.

Those are just a few of the reasons I've seen. But they'll print each revision on different colored paper, and by the time shooting starts you'll typically never be going off the original white script.