I would like to dedicate this post to my beloved dog.
The animal exercise was first introduced by Lee Strasberg as part of the actor's training. It is taught today to acting students as an essential tool toward a deeper physical characterization of a role.
The exercise consists in the exact reproduction of an animal's behavior: the actor imitates the sounds of the animal, moves exactly like the animal, and reproduces its general attitude. What Lee Strasberg suggested, though, is not to begin right away with a pure imitation. The actor should, instead, start by carefully observing its behavior to the tiniest details, asking questions like, "In what does this animal differ from me?" or "What makes it pull its ears back in certain contexts?".
Little by little, after weeks of practice, the actor gradually stands the animal up. At this point, the animal element is adapted to the human being, retaining its strength but becoming much more subtle.
The animal is chosen according to the characteristics of the role to be played.
Good actors often use animals to build their characters: Marlon Brando disclosed all his brutal manners in A Streetcar Named Desire by moving like an ape; Jim Carrey walked pulling and pushing his neck like a pigeon to portray his egocentric and eccentric mode de faire in Ace Ventura. Shall we mention Batman? Tarzan? Mowgli? Or Catwoman?
Personally, I came to realize years ago - so, even before becoming acquainted with the animal exercise - that living with a pet for countless years implied a very close, daily, and effortless observation of an animal's behavior. When you live with a pet, you become familiar with its different ways of barking or meowing; each way has a different meaning. Wagging the tail has meaning, but not always the same meaning: my dog does it when he's happy, or when he's asking for something, kind of like buttering us up because he knows he shouldn't ask us for that. The way he looks at me varies with the context. He bit me once, and a second later he looked at me in a way that could only mean, "I'm so sorry, I don't know what came over me".
I believe that the animal exercise will be much, much easier if the first approach involves your own, dear pet. The process of observation and study is much less intentional when it comes to your pet. You will realize how easy it is to imitate him and, at the same time, you will feel perfectly comfortable, natural, and subtle when adapting its behavior to a common "human" context.
I also believe that a similar study of animals - maybe, but not necessarily carried on in a different way - could also benefit a writer's work on fictional characters.