This book came out in 2012 (or 2008, depending on the edition). It is refreshing to see that Swain has updated his writing style and hes stance on race and gender to a 21st century standard. Much of this material was covered in a more condensed manner in the book for which he is primarily noted--Techniques of the Selling Writer--but it goes down easier.
There are dozens of books about character creation, some of them much more profound than this one, but the idea Swain presents that is particularly useful to me is the notion of assigning labels to characters, and fleshing out the labels with tags, traits, and relationships. This produces characters that are vivid and distinct from each other. It gives the author a handle on the inner nature of the character, and gives them a consistent framework for portraying the character to the reader.
In Swain's epistemology, a Label is the dominant impression the character makes upon the reader. A label is a four-plex of age, sex, vocation, and manner. For age, Swain refers to adjectives like young, adolescent, mature, old. For sex (to which in this third decade of the 21st century we would probably refer as gender) he means descriptive adjectives that imply rather than state the gender. Vocation goes beyond traditional occupations, and extends to other social roles, e.g. housewife, bum, significant other. Manner is the individual’s personal bearing—their habitual stance and style.
Applying this to one of my protagonists, I would label her the "angry, middle-aged, sorceress."
Swain then suggests fleshing out the character with Tags, Traits, and Relationships.
A tag is a limited, specialized label. A name or a nickname is a tag. Categories of other tags are: appearance, ability, speech, mannerism, and attitude.
Appearance is any item that strikes a distinctive note. Swain suggest two or three items per main character.
Attitude is habitual behavior patterns exhibited when a character reacts to particular kinds of situations. Racism, sexism, sanctimony, suspicion, anxiety, discontent are examples of character attitudes which are expressed in their behavior. =Behavior= is the key, here. Only you know what the non-POV character is thinking. You can only show it to the reader through their behavior. Even for the POV character--they are probably blind to their own biases. Show it through their behavior.
Moving on to Traits. A Trait is a character’s habitual mode of response or pattern of behavior. Traits as an aspect of character are abstract and general, but behavior is concrete and specific.
A relationship is the way a character interlaces with others, their associations with others and reactions to the people with whom they deal or come in contact. Each if these dealings and contacts are different, depending upon the nature of the relationship.
So, fleshing out my example protagonist with Tags, Traits, and Relationships--my protagonist is an "angry, middle-aged, sorceress, whose locks of unbound auburn hair writhe like serpents, but who has come to fear that her husband--a powerful magus--has come to disrespect her." Boy Howdie! is she pissed. Her rage at his disrespect drive the first third of the novel.
I don't know how well this will sell, and I don't know how useful Swain's advice will be, but I'll let you know as a comment to this post.