He needs to show up in the parking lot scene before the end of the book and threaten Murphy with the knife. This is where different-colored ink comes in handy. When you finish your read-through, go back and start adding pages. Don't do this before you've done one complete read-through, because as you get to the end of the book, you'll find places where you jumped to conclusions that you'll need to support earlier in the book. Finally, re-read the book in manuscript form one time after your major edit.
Writing uses a lot of paper, a lot of toner, and a lot of time and patience.
But the end goal is to get your manuscript in front of an editor who wants to buy it, and in order to do that, you have to demonstrate professionalism. Don't edit, and then forget to check your changes. For the read-through of your manuscript, you can print it in small type with small margins and on both sides of the paper if you want. You're mostly going to see if you've introduced any new mistakes with your new sections. I have discovered that I always add a few spelling mistakes at this stage, and invariably I'll take out part of a sentence, edit the rest of it . . .