With his round head, wobbly smile and curlicue hair, Charlie Brown is perhaps one of the easiest cartoon characters to draw — and the hardest to animate.

In fact, for “The Peanuts Movie,” it’s the character’s very simplicity made the process so challenging for director Steve Martino and the team at Blue Sky Studios, the same folks who had previously found a CG style suitable for Dr. Seuss’ “Horton Hears a Who!”

But what came so naturally to Peanuts creator Charles M. Schulz doesn’t translate nearly so organically to computer animation, where Maya and ZBrush software is coded to apply the laws of physics to the way objects look and move. In the real world, a character’s head and body remain consistent, whether seen from the front, back or side — whereas Schulz took certain artistic liberties, changing the position, shape and volume of Charlie Brown’s features (eyes, ears, nose and neck) according to whatever direction he might be facing in any given panel.

“With a pen, you can do whatever you want,” says Sabine Heller, character development supervisor on “The Peanuts Movie.” When animating via computer, however, everything is limited by the rigging process, which establishes a sort of virtual skeleton by which an animator can manipulate the character the way a puppeteer might.

As on past productions, Martino’s team began by building a base character model and equipping it with a rudimentary rig. “Then we give it to the animators to break,” Martino says. “We ask them to start pushing extremes, and we learn where it gets ugly.” A crease could appear across the side of the face or an arm might bend oddly when it rotates. Then the rigging team goes to work fixing those problems, while remaining true to the way Schulz had drawn the character for 50 years.

In the case of “The Peanuts Movie,” Blue Sky spent roughly two years perfecting Charlie Brown’s rig — a period rarely seen in cartoon R&D, but absolutely necessary for a film whose characters had to match preexisting designs. According to Heller, “He was the prototype. Everybody was inherited from Charlie Brown,” adding animatable curls for Frieda or the ability for Schroeder to hunch over his piano without becoming twice as tall as the other characters.

“When somebody makes a Charlie Brown sculpture or a Snoopy toy, there are certain perspectives that look really good, and there are others where it looks off,” Martino says. “We learned early on that Charles Schulz only drew the heads in six poses” — direct, head-up, left profile, right profile, ¾ left and ¾ right — and any other view can have a jarring effect.

Still, that didn’t stop Blue Sky from trying to build an all-purpose Charlie Brown rig, one that could be shot from any angle — but ultimately had the unique quality of looking slightly “off” no matter which way the camera was facing. Scrapping that plan, Heller’s team eventually decided to develop a much more complicated system, a rig with a control that allowed animators to automatically switch among the six different views.

“The rig would automatically move the features around so it would match the way Charles Schulz drew the character,” Heller says.

In some drawings, Charlie Brown has just three fingers, while in others, he has five, so the rigging team gave animators the ability to flip back and forth without making him seem freakish. They also made it possible to turn on multiple limbs for every single character, allowing for an old-school style of motion blur, in which several arms or feet appear in the same frame — as when Charlie Brown falls off the ladder in the library or Snoopy dances the flamenco.

Snoopy’s rig presented other headaches in that Schulz seldom rotated the dog’s face, keeping the same silhouette whether he was looking forward or in profile.

This posed a unique challenge for animators: How do you suggest a head turn when the only thing moving is Snoopy’s nose? What to make of a smile that seemed plastered on? And how to explain the sudden appearance of a second eye where only one had been there before?

Though Snoopy’s rig was designed to flip between poses, it fell to animators to cushion the transitions. Typically, they would use a blink to introduce the new eye, while shifting the body ever so slightly to convey movement.

“Snoopy is the ultimate Picasso challenge,” says Martino, referring to the subtle optical illusions they discovered in Schulz’s inadvertently abstract drawings, complicated by the addition of dimension, texture and lighting. On the one hand, giving Snoopy plush-like fur subtly tied in many viewers’ associations with stuffed toys, but posed other obstacles.

“We came up with all kinds of new technologies,” says animation supervisor Scott Carroll. “If you think about Snoopy’s eyes and nose sliding around, that fur gives you a landmark, so we had to be very judicious in how we moved them, and then also figure out a way to make sure that (his features) felt integrated into the fur.”

To help disguise the fact that the characters were popping from one position to another (effectively eschewing the smooth in-betweens traditionally seen in CGI and behaving more like stop-motion replacement models), Martino actually downgraded from 24 frames per second — the speed used in past Blue Sky features — to a slightly jerkier 12 fps approach, known as “animating on twos,” used in more cost-conscious animation, including the classic Charlie Brown TV specials.

“It’s a snappier style, and I like it,” says Martino, citing specific tricks the approach allowed — as when Snoopy is typing his novel and Woodstock whips out a red pen: “The style of animation is such that Woodstock can reach behind his back, and in the next frame, he’s holding a pen that’s nearly as big as his body. It allowed us to dispense with pure physical logic and take on an approach where we’re thinking like 2D animators.”