you can't tell a book by its cover

You can't base your opinion of something (or someone) on the way it (or one) looks. A variant of "don't judge a book by its cover." This report may look dull, but you can't tell a book by its cover—I will have you riveted by the facts and figures in no time! I know she's a little frumpy, but you can't tell a book by its cover, man!