I would say that the writing "too much" isn't really too much.
When I was at college the class was split into two production crews and told to make a 30 minute TV show. (It was a TV & Video production class)
My group was filled with just an amazing group of people and we had so many ideas that we just had to cull them down. The other group has this really cool concept, but they very clearly only had about 20 minutes of material. They worked it, but it showed that they were stretching and padding. On the other hand, ours finished and the reaction was "It's finished already? I want more."
I think, my point is that if you have written a script and you are seriously lo on page count, don't look at places you can add to, scenes you can lengthen, that kindof thing. Instead, go back to the start of the writing process for that project, look at your characters, look at plot, look at subplots and start to build it again but with the extra material.
It really is like building. And if your wall isn't high enough, it's really difficult to try and force bricks into the existing structure and still retain the integrity. You have to go back down to the foundation.
That old nugget that writing is rewriting is a cliché, but sadly, it's true. It can be frustrating when you start out on this journey, but (and I NEVER thought I would feel this way when I started) I have found I like the rewriting part. That's where the skill comes in. That's the challenge. The hard part.