Proofreading is not just spelling and grammar

A few days ago I was asked what job I did for a living, and obviously I answered that I was a freelance Proofreader. The reply I received was a little unexpected as I was told it was an easy job as all you need to check is spelling and grammar.

Yes, spelling and grammar must be checked, but there is so much more to proofreading than that. You have:

Tenses
Repetition
Consistency of spelling
US or UK spelling
Other foreign spellings (including accents)
Tone
Relevance and Clarity
Highlighting phrases that could be controversial
Reference checking
Description checking (I once proofread a book where the heroine started with blue eyes, but finished with brown ones)
Checking calculations (returning to my days as a Finance Officer)
Suggesting format improvements

You need to be on the ball. Every good author needs someone to make a final check.