You've said you warn people that you can be a real jerk
when you edit their books. What do you mean by that?
And for the record, I don't say I'm a real jerk. I say I'm a real bastard. Perhaps it's a semantic difference, but I think it's worth stating.
That means my job isn't to find what works and praise you for it, but to find what doesn't and help you make it better -- even when, and especially when, it hurts your writer's pride and bruises your ego. Then I prune and cut and slice and dice and point out all the ways your story doesn't meet the high standards that both you and I have for it.
Then we work together to make it better.
I think it most typically shocks people who come to me expecting glorified proofreading. What they get back are tracked changes that often bleed more red than the original black.
I'm ruthless. I go after weak verbs like a serial killer, find ways to murder adverbs and most adjectives, make sure your characters have consistent personalities and speech patterns throughout the book. I destroy your redundancies when you overuse your favorite go-to words in a single page, paragraph (or sometimes even sentence). Recently I even rewrote an author's opening paragraph so that it started with something to draw in the reader and stop wasting time on all the pretty flowers.
Grab me first, flower me later -- this is a novel, not a date.
Perhaps that why I don't typically edit the work of my friends. Though it is telling that people whose work I do edit usually become friends after the first good cry. *grins*