I was recently asked to do a quick polish of an animated feature film (yes, it's got a penguin as the protagonist, but it was started before the current penguin craze really got underway). A newer writer to whom I mentioned this asked me what the process is when you're doing something like this, so I thought I'd share it, especially since it applies to rewriting your own material as well:
1. Start with the big picture. Read the whole thing without taking very many notes, just to get a sense of how the material flows and whether there are any major gaps or problems. In the one I've been working on, the main problem was that we (the audience) don't understand the motivation of the villain until almost the end. We'll care more if we know much earlier what he has in mind.
2. Fix any structural problems first. They will probably have implications for a number of scenes. In this script, I added an early scene in which the villain's plan is revealed to the audience.
3. Still dealing with the big picture, make sure that the characters all come across distinctively and memorably. This script has a larger-than-life diva in it, but she wasn't "big" enough in her dialogue and actions. Making her more extreme made her funnier.
4. Now go through it again and check whether the locations and the dialogue are doing their jobs: supporting the action, revealing the characters, and moving the plot forward. For dialogue, how the characters speak should reflect who they are, and set them apart from each other.
5. If you're being a script doctor for someone else's script, remember to serve the needs of the client. You're trying to help them get the script the way they want it, not necessarily the way you would have written it yourself. Often there's a difference. (Hint: don't start by changing the names of all the characters!)
You also have to be aware of how much leeway you have in changing things. My first script doctoring job was on the feature film, "Mannequin," and I couldn't change too much because they were starting to shoot the following week. In the current project, they've already done a lot of pre-production work, storyboards, pencil tests, etc. so they want to keep as much of the current script as possible.
I hope that's useful. I go into the subject in greater depth in my book, "Your Writing Coach," published last month by Nicholas Brealey Publishing and available from barnesandnoble.com and amazon.com/co.uk as well as shops.