We all know what Let’s clean out the garage means.
In case you don’t, it means that the person making the suggestion wants to get rid of a lot of stuff the person hearing the suggestion wants to keep.
I edit while I write. I know that’s wrong, but I don’t care. For one, it keeps stuff out of the garage that somebody else will one day tell me to throw away.
Otherwise, editing begins when your manuscript is on the home stretch. At this point, you’re the one telling your characters it’s time to clean out the garage.
Basically, they don’t want to do this unless it means giving them more lines. Usually, it means giving them fewer lines or (worse yet) fewer scenes. They (the characters) like the garage as it is because that’s all they know about each other and themselves unless you’ve used them in previous books.
Characters become restive, even combative when you bring out an editor’s broom and a toll of trash bags (the novelist’s “cutting room floor”). They want to story to stay the way it is, go to a publisher, show up on the bestseller list, and make them as famous as Madame Bovary and Captain Queeg even if readers think they’re crazy.
The madame and the captain had some great lines, they’re in a box somewhere, but they probably don’t belong in my book. If I find anything remotely resembling either character’s habits or lines, they need to go.
Gandalf had some great scenes, such as when he fought the Balrog in The Fellowship of the Ring. If anything like that’s in this garage, it’s got to go.
One of the mysteries of editing is that you’re not simply throwing away trash, you’re throwing away some darned good stuff. Goodness knows, your characters want to keep that good stuff. So do you, probably, but if you ponder it long enough, you’ll know its not a good fit for your story.
(What you don’t want is reviews that say “Snoopy’s Sopwith Camel vs Fokker dogfights with the Red Baron sure were exciting, but why were they happening in a small town in the Florida Panhandle in 1955?”)
So, this is where I am with my novel in progress. The characters are saying, “Let’s blow this joint!” (and we’re not talking about marijuana here) while I’m offering them a bigger share of the $10000000000 adance if they stay around and play nice while we get this manuscript ready for the publisher.
Plus, I keep telling them that my publisher goes flat nuts if I send in something that belongs in the garage sale bin, and “trust me, you don’t want to be around when that happens.”
Basically, I think cleaning how the garage is the worst part of homeownership and editing is the worst part of writing a novel. Others’ opinions may vary
Malcolm's Round Table 
I’m actually a better editor than I am a writer.
My characters have learned to suck it up and live with it.
I always figured that if I told me characters to suck it up, they would run off into the woods.
I love editing – once I’ve got started. It’s the opening of the garage door (to stretch your metaphor further) that is difficult. But I know I shall love the cleaned out garage when it’s done.
Unlike the garage, once you’ve edited your garage stays cleared out and tidy!
I usually keep a box of Bits Too Good To Throw Away under the counter somewhere 😉
I often keep those snippets that don’t fit in a file for potential future use–which never seems to happen. I don’t think I’ve ever lived in a house with a tidy garage; so I don’t trust it.