But perhaps the most impressive special effect in the top-grossing superhero flick was the Hulk, an ILM construct that built 1,400 pounds of muscle-bound CGI freakiness around the monster’s mild-mannered alter ego Bruce Banner (and actor Mark Ruffalo). ILM’s Jeff White told Wired that job number one in building a better Hulk was building the elements of character within the CGI, and shared five key steps that helped them create the most impressive visualization of the Hulk to hit any screen, big or small:

1. Give the Beast a Pot Belly. “This Hulk actually has a little bit of a muffin top. We gave him a little bit of bulge above the beltline because Joss told us he didn’t want somebody who was zero percent body fat. He wanted more like the old TV show Hulk, where it looks like natural human skin. What’s great about that is that gives us something to add all these secondary dynamics onto, because we can simulate with spring systems all the fat layers jiggling around and skin sliding on them.”

2. Humanize the Eyes “Joss Whedon’s directive, a semi-controversial move, was to incorporate [actor] Mark Ruffalo into the design of the Hulk. He wanted to see Mark in there, especially around the eyes. Even though the Hulk’s eyes are green, they really match Mark’s quite well.”

3. Get the Actor in the Dome “We captured a phenomenal amount of data from Mark,” says White, describing ILM’s motion capture process. “First we did a full life [foam latex] cast of his face, which is not that fun. Then we [put] Mark in this big dome with 611 lights in it. Using a high-speed phantom camera, we fire off echo light individually. And what we get back is footage where we can see Mark, with lights firing one at a time moving around him — front light, back light, side light, rim light. That’s all incorporated in there. Our very first digital version of Mark was scary. It did not look like Ruffalo at all. It’s one of those deceptive portraiture things where it takes a while to get there. But once we felt we could digitally recreate Mark Ruffalo, then we resculpted that exact same geometry into the Hulk and all the textures went along with that.”

4. When in Doubt, Go to the Face Although the Hulk performs plenty of feats that defy human physics, ILM still kept the human emotional element in the picture. “Even when the Hulk does very non-human physical things like jumping between buildings, we had Mark give the kind of facial expressions he would do if he were exerting himself to that level. That gave the animators a great template so they could really tune those subtle performances to arrive at the final result.”

5. Gorilla Gorilla Gorilla “Joss had lots of comic book poses as references that liked, but he also had gorilla [photo] references that we incorporated,” White says. “This … gave the Hulk a very cool look. And also, he needs to stand in the frame with the rest of the Avengers, but the Hulk is more than eight feet tall. That makes for a very awkward composition, so bending the Hulk over and having him in that tensed, ready position helped us compose the shots better as well.”

Ultimately, White notes, “Everything about the Hulk was grounded around the actor.”