Another way of looking at the exercise Donald Maass includes in his book Writing the Breakout Novel -- that focuses on helping you enlarge the qualities of your protagonists/characters -- is to think how you can make your character unforgettable. Who hasn't heard of Hannibal Lector, Harry Potter, Frodo Baggins, or Leopold Bloom? By adding tidbits of humour, action, internalization, by heightening what s/he says, does, and thinks, you give the reader a character that will stay with them, in a story where every moment counts. Take a violent situation and tone down your protagonist's response. Take a dark scene and add humour or emotion. Add a line of dialogue that is totally unexpected. Make the changes to your manuscript. Give your reader a character they'll never forget.