Can true love exist between a circle and a square? That’s the question at the heart of Pixar’s “WALL•E,” at least as far as the designers behind the animated robot romance are concerned.

“He” is a boxy little trash-compacting droid named WALL•E. “She” is a smooth, oval-shaped scout named EVE. Neither character can talk, but they were engineered to compliment one another’s differences, says lead character animator Angus MacLane, who explains the fundamentals of their design.

• Begin with a box

Shy but curious, WALL•E is like a boxy turtle who can tuck himself away for protection. “Originally, it was meant to be a space-saving idea,” MacLane says: The film’s eponymous Waste Allocation Load Lifter-Earth Class droid was one of countless little robots designed to clean up after the human race. The units were packed by the hundreds into enormous trucks, so it was important that they could fold up into cubes and stack neatly.

According to MacLane, the ideal model was more R2-D2 than C-3P0. Director Andrew Stanton (“Finding Nemo”) was adamant that WALL•E should have his own body language, which meant avoiding “dude-in-a-suit or Fozzie Bear acting.” Rather than force the robot to mimic human gestures, Stanton felt WALL•E should be constrained by the boundaries of his design.

“With R2-D2, it’s more about the limitation of how just a head turn can mean so much,” MacLane says. “Instead of having a robot that was puppet-y and moves around a lot, our goal was to use a more minimal sense of acting. He naturally looks kind of sad.”

• Give him functional “arms” and “legs”

In order to perform his tasks, WALL•E needed a way to move around and collect trash, so the designers gave him tank-like treads, which suggest his connection to such 20th century inventions as the bulldozer and forklift. “We knew that his chest was going to be the compacting area,” MacLane says, “but the real challenge was how the arms could move the trash into the body and also serve as acting tools.”

Stanton set a “no elbows” rule, which created headaches trying to figure out how WALL•E could reach the buttons on his own chest if his arms didn’t bend. The team considered flexible Doctor Octopus-style arms (too elaborate) or limbs that telescoped like a car antenna (not strong enough) until MacLane found the solution in his father’s former profession designing inkjet printers, which operate using a slide-like mechanism.

By allowing WALL•E’s shoulder to move along an L-shaped track, the animators found they could subtly tweak the character’s posture. “Anytime he’s around EVE, we put the shoulders low and the hands close to the chest, which made him much more unsure and nervous looking. But whenever he’s being the mechanical robot, his arms are back near his neck to make him more functional and competent,” MacLane says.

• Add eyes for expression

Until now, virtually every Pixar character has boasted bright human-like eyes (even the cast of “Cars” featured glistening orbs set against expressive white windshields). But WALL•E is different, a throwback to the faceless Luxo lamp featured in the company’s logo.

His head shape was inspired by a pair of mechanical binoculars, which bend and flex to convey different emotions. Of course, some expressions came more naturally than others. “It’s hard for him to be mad,” MacLane says. “When his binoculars lock together, that forms a straight line across the top of his head, and at most he looks irritated. But he can never be really pissed.”

And because WALL•E could say nothing more than his own name, that shifted the acting burden to his eyes. In closeup, his irises dilate and the lenses shift back and forth. Small levers on either side of his head pop up to serve as eyebrows when needed, although the animators tried to use them sparingly. “If you’re having WALL•E react to everything with equal emotional intensity, there’s no emotional range,” MacLane explains.

• Now imagine the opposite extreme

By contrast with WALL•E’s rust- and mud-covered industrial machinery, EVE is a polished white Extraterrestrial Vegetation Evaluator graced with higher-level functions. Her round Apple-like stylings are “no coincidence at all,” MacLane says. “It’s a very conscious nod to sleek design.”

Encased in a seamless white shell, EVE hovers above the earth, an angel with a hidden laser blaster. “She’s a bit like a matryoshka [nesting] doll, in that she starts out like this impenetrable egg, and then she unfolds to be a little bit more vulnerable, but functional,” he says.

EVE’s head and arms are held in place by magnetic fields, rather than the clunky joints of her earth-bound love interest. “She’s looks dangly and wind-chimey when she’s in automaton mode,” MacLane explains, “but when she gets emotional, she does more arcs, like a porpoise flying.”

• Create a complimentary personality

Instead of a face, EVE has a black screen on which blue LEDs display her various emotions, the expressions inspired by the iconography of classic cartoons. Upside-down U’s convey laughter, an anime convention that suggests nonexistent cheeks pressing up into her eyes. A line running straight across indicates when she’s mad, like the stern brow of a superhero, symbolizing geometry and facial structure she doesn’t actually possess.

And while EVE’s exterior is smooth, some of her graphics describe a mechanical process going on internally. Her lights flicker at the end of each idea, blinking to begin a new thought process. When her casing unfolds, the seams in her design begin to show, and yet, she is clearly more advanced than WALL•E.

For MacLane, that was one of the appeals of their mismatched romance. “I think by not being afraid of a character who is somewhat normal, there’s a lot of relatability that is completely essential to the storytelling,” he says. “WALL•E represents the notion of who people actually are: The guy has dreams, and he has chores; he’s not the best at everything he does, but he’s good at his job. Just as a superhero team would have a bruiser character, that would be WALL•E, whereas EVE is a much more aggressive hero.”