What is empathy and why, according to Brian Cleeve's essay "The Value of a Wart," is it necessary for creating a realistic character? What are the techniques necessary to make a character “spring alive” from the written page according to Cleeve?

Empathy is the ability to understand another person and to put yourself in another person's shoes. Having an empathetic understanding of others is necessary to create real, full characters, as authors need to understand an entire person, warts and all. For example, in the author's view, if a character has a wart, as the Miller does in Canterbury Tales, he won't be as confident. A writer should understand the totality of his or her characters and place him or herself in their shoes to make the characters seem alive. 
Only you can write about which characters spoke to you from your reading, but, in general, characters who come alive seem human. They are not mere cardboard cutouts. Instead, the readers understand their motivations, dreams, and quirks. For example, if a character is mean, the character might seem wooden if we don't know why he or she is nasty. For example, is the character insecure, unlucky in love, feeling ill, etc.? As Cleeve writes, a character comes off the page when we as the readers can identify with that character and relate to him or her. If we understand that character's motivations, we can begin to empathize with him or her and feel the character's humanity.