I've spent much of the last 6 weeks going back through the novel, teasing out scenes here, rewriting them at times, cutting in others. What has struck me and my readers is that I often build to a climax, and then avoid the climax and move on. Which means, when I go back I need to build all these scenes to their natural conclusion: which is truly exhausting work - and probably the reason I avoided doing it the first time round. I think of this process as a little like casting a spell, which is how Harry Potter comes in: because hitting these scenes is just like spell-casting.
Preparation: think about the scene, work out where you want it to end up, and what the climax should be. Get all the ingredients you need with you (not spider's legs but tea of coffee or cigarettes or whatever you're going to need for this process which might take anything up to a couple of hours.) Sit down. Clear away all the detritus of the day. Focus your mind.
Casting: Focus clearly. Put yourself into the characters - in wizard terms, possess your character's body and feel how they feel and see what they see and write it down as clearly and honestly as you can. There will probably be more than one character that you have to possess: which is an exhausting process as you take on the personality and backstory and experience of each of the characters in turn. Like a spellcaster, you are summoning spirits and illusions and movement to enchant and entrance the reader. Focus: your are summoning a world and a moment into being.
Recovery: If the spell has been cast properly then as all wizards in all tales - you will be exhausted. You're body will feel empty and dislocated from the world around you: because in truth you have passed from our world into the world of your imagination; have possessed the body of imaginary people and moved them like real.
The spell ends.
This analogy becomes much more understandable when you think of the original meaning of the word 'spell', whose spelling (from a different word source) hasn't changed since Old English times, and which comes to us in the word 'gospel' [O.E. 'godspel'].
For the original meaning of 'spell' is story. And what else is story-telling than the working of illusions, the charming and enchanting of an audience: once through sitting in a fire lit hall and telling them a tale, but now more likely through the words we write on a page.