When Edgar Allen Poe published “Murders in the Rue Morgue” in 1841 the murder mystery was a relatively new genre. He wrote a few more of these increasingly popular detective stories before leaving behind his own mystery. In 1849 he was was found wandering injured and delirious through the streets of Baltimore. The brilliant writer never regained his wits enough to explain what had happen to him. He died a few days later leaving behind a real unsolved mystery.
Current detectives have more techniques for identifying possible suspect but still lean heavily on the use of a psychological profile. You are probably familiar with a few of the terms they throw around, such as calling card, MO, and motive. These are not just reserved for suspects in murder mysteries. Fictional characters take on their own personalities when you remember to consider each of these features. Paying attention to these aspects doesn’t make characters predictable (otherwise criminals would be easier to catch) but does make them less confusing.
The calling card is a quirky behavior, an eccentric ritual that goes beyond what is needed. This is also called the signature aspect in criminology because it comparatively unique. It provides an insight into motive and is derived from a deep seated psychological need. (Major characters should also have unfulfilled psychological needs unless they are robots.) For example, a woman who had to grow up too fast in a distressing family situation may show a propensity for collecting stuffed teddy bears in an attempt to regain a lost childhood. Another from a similar background may compensate by calling her own children on the phone, every day. Signature behavior develops uniquely for each person based on personality, motive and MO. It may increase or decrease but doesn’t really change.
MO or Modus operandi is Latin for method of operation. This is a character’s preferred way of interacting with others. Consider two different teenage boys in their attempts to attract teenage girls. One may decide a show of physical strength, such as pelting a rival with a football, is the way to gain attention from the fairer sex. The other, who uses his wit may point out the disproportional number of felons in the NFL after the pelting incident. Modus operandi is the preferred method that people use to reach a goal. It is not fixed, but based on learned behavior and changes over time as the character gains confidence through experience or descends into neurosis due to stress. However, there has to be a reason, a point in time or an event that you can put your finger on, that causes the shift in MO.
Finally we come to motive. We may never understand why real people do what they do, but we expect to be able to detect motives in fiction. The reasons that drive characters to act may be initially hidden but should be revealed as the story progresses. A character’s motives can be transformed but this calls for an event with much more earth-shaking than that needed for developing a new MO. Just like the calling card, internal motivation tends to remain constant, unless the character goes through trauma or brain surgery. The calling card can provide a window into the character’s motivation if you are subtle about connecting the two. Readers often enjoy uncovering this on their own. After all a major point of creating the story is for the reader’s enjoyment.