Who’s a good boy? In movies, the answer is often a dog with soft eyes, a wet nose and ineffable charm. Sometimes the dog is a puppy bounding with joy; on occasion its gait is uncertain and muzzle touchingly gray. The movie dog can be just part of the scenery, though on occasion it delivers a moral lesson. Dogs can be comic, lovable, heroic or tragic; much depends on their humans because, while man rarely bites dog, he does execute unspeakable abuse on his ostensible best friend.

Lately, I have been thinking about animals in movies, the creatures that rarely make it on camera and those that do. So, of course, I have been thinking about dogs, the big screen’s favorite animal. Top dogs, underdogs — the movies are filled with Canis familiaris. Consider the cape-tugging C.G.I. engine of chaos, Krypto, in James Gunn’s “Superman.”

The moment Krypto bursts into the story, you know this isn’t the broodingly dark Man of Steel but a kinder, sweeter Superman. As his name suggests, Krypto is Kryptonite of a type — he slays metaphorically, and delightfully.

This is Krypto’s purpose. Like other movie dogs, he runs to the rescue — to help Supes — but also rolls on the ground as if enjoying a laugh that we share in. That’s the thing about movie dogs: They seem so easy to read, which makes them ideal vessels for conveying ideas like happiness, fear and aggression. Cue the wagging tail, the bared fangs. As in real life, screen dogs are obedient, biddable, though also comically or scarily out of control.

They can be dear friends …