Do your characters have goals? I'm not talking about your main character. We all know that he or she must solve the mystery or fall in love and resolve issues, etc. I'm talking about the other characters in your book. To be interesting, they must also have goals -- some good and some bad. If they don't, we (your readers) won't care about them.
Think in terms of your own life. You have goals (finishing your book for one). And the people in your life have goals. Now think about those people in your life. What makes them interesting. It's not that they have a goal to make it through one more day. It's that they dare to dream bigger than the life they have.
So my recommendation is to do the reader a favor and punch up the interest level on secondary and even minor characters. We'll all love you for it.
What do you do to make your characters interesting?