The proofreader is (usually) the last line of defense before a book or article goes public. Yet, too many errors slip through. Sure, some inevitably escape the trained eye. We're human after all and prone to error. However, the vast majority of them ought to be stopped and pointed out by the proofreader.
So many folks blame the author or editor for these slips, but, as an author and editor, I've seen first hand how easy it is to insert errors while correcting other ones.
The author and editor are too close to the manuscript, making it possible to miss the details after a while.
A proofreader brings a fresh set of eyes to the piece. There's no emotional attachment or weariness from having written and rewritten sentences, choosing more descriptive words, or cutting out well-love scenes that add no value to the story. But, unless attacked with the determination to find those pestersome errors, proofreading will prove less than adequate.
How to Proofread
Avoid getting too swept up into the story. It's great if the proofreader enjoys the manuscript. But, don't lose sight of the reason for reading it in the first place. Remaining focused on the task a trained mind. Definitely, let the editor and author know the manuscript was fun to proofread.
There's more to this
Why proofread aloud?
The brain has a way of correcting flaws during silent reading. Because of this, speed reading is a bad idea for a proofreader. Reading aloud exposes the tendency of the brain to auto-correct. I've had to reread sentences when what my brain perceives differ from what my lips said. That's when I'm able to catch and point out the issue. This helps the author release a manuscript with less errors which results in less embarrassment and enhanced reading pleasure.
So, take proofreading seriously. It's important!