I edit novels (among other works). I was having a discussion with someone (not an editor) who didn't understand my technique. 

What I do is read through the document, and the moment something occurs to me — whatever reaction I'm having for good or ill, whatever I catch, any questions I have, mistakes I spot, delightful turns of phrase I notice — I mark it. When I get to the end, I then go back and briefly review my remarks, so that if I wrote something like "Is there a reason that Dave went to the deli?" and I find later that Dave needed to be at the deli so he could overhear Sarah, I can delete that comment. 

My friend thought that I should read through the entire book first, *as though I were reading it for pleasure* rather than as an editor, and then do a second round as an editor. He thought that reading it first as an editor was somehow unfair to the "spell" that the writer was trying to create with the book. 

I really think that it's important for a writer to get my first-read impressions. If I already know that we aren't going to return to Blandings Castle for another 150 pages, then I don't feel impatient. But if I'm reading through for the first time, as a reader I'm wondering "when TF are we going to get back to Blandings Castle already? It's been over 100 pages!" I think the author needs to know that the reader is feeling impatient. If that's deliberate, or if the author doesn't care, that's a legitimate choice. But the writer can't know that unless as a reader (and editor) I give her that feedback.

So I ask any other editors here: what's your technique? Mark as you read on first draft, or read and then mark? Or something else altogether?