One of the most hyped scenes in Taika Waititi's Thor: Ragnarok is a gladiatorial battle on the planet Sakaar between Hulk (Mark Ruffalo) and Thor (Chris Hemsworth).

Although Hulk is a CGI character (with Thor also being digital at times, too), the fight feels real and visceral. And there's a reason for that: not only was it staged with the principal actors, but the stunt doubles were specially chosen to represent the appropriate scales of the towering Hulk and the shorter, but still powerful, Thor — in a way you might not expect.

The visual effects crew behind the work break down how they accomplished the dramatic fight for SYFY WIRE.

Staging a stadium showdown

In previous Marvel film fights involving Hulk, the big challenge was to represent the huge character on set with either Ruffalo or a stunt performer stand-in (who would be replaced later with the CGI character). Often Ruffalo or a stunt double in motion capture gear would wear a backpack rig that added extra "bulk" and even a cardboard cut-out head while taking aim at an opposing actor or stunt double.

But according to Thor: Ragnarok's overall visual effects supervisor Jake Morrison, "that would often end up just being a bunch of guys throwing punches at each other, like a bar brawl. There's a limit to what you can do interaction-wise because the other guy is just wearing a stand-in rig on top. You can't really wrestle that well."

The solution was to re-think the problem and do some "reverse engineering math."

"I went, look, if Chris is 6'4 so Thor is 6'4 and our Hulk is 8'6, that's a certain percentage scale up," says Morrison. "What if we do it the other way around, and say that the Hulk is actually 6'4, what's the reverse math to this — what could be the smallest Thor we could shoot?"

That appropriate height turned out to be 4'8 or 4'9. The next question was finding a stunt performer who was around that tall. Luckily, one existed by the name of Paul Lowe, a well-known stunt performer with credits such as The Dark Knight Rises and The Mummy Returns, who regularly doubles for children. Standing in for Thor, Lowe battled other stunt doubles for Hulk. Because of the height difference, Lowe would have to punch upwards (just like Thor would to Hulk), or if he kicked Hulk it would hit the character's knee at just the right height.

Stuntman Paul Lowe posted these images on his Twitter account from the Thor: Ragnarok shoot.

This part of the fight was motion captured, and followed beats set out by storyboards, and by previsualization from The Third Floor. Later, Chris Hemsworth would repeat the actions on a bluescreen against heavy bags and focus pads. "But now we knew the height at which the punches had to be," notes Morrison, "and whether he had to jump up and leap onto Hulk's neck. We had built pieces of rigging to represent Hulk, too, and even painted them green so the right light bounced off onto Chris."

When other footage of Hemsworth, a motion captured Ruffalo, stunt doubles, and the earlier fight pieces were ultimately brought together — care of visual effects from Industrial Light & Magic — they clicked. "ILM would embellish the scenes, and their animators really just ran with it," explains Morrison. "They did incredible stuff, and we had such a strong foundation for the fight to begin with."

Same Hulk, but different

Hulk has been stuck on Sakaar for some time and in permanent monster form, instead of Bruce Banner. That gave the filmmakers scope to present the CGI character with a slightly new look. "ILM went back to first principles and rebuilt the Hulk from scratch," says Morrison. "We knew that we needed the fidelity of the model to be much higher to be able to do all this spatial animation and deliver dialogue."

For the re-build, ILM went back to Marvel Studios' head of visual development Ryan Meinerding. "Ryan continued developing Hulk's evolution, and really put an emphasis on making sure Hulk felt appropriately heroic which was reflected by a more chiselled and athletic Hulk with the square jaw of a champion," outlines ILM visual effects supervisor Chad Wiebe. "Hulk's hair groom also changed to something more Sakaarian, compared to the shaggy beast in the Avengers films. His color and skin were also updated, and we added scarring from previous arena battles, along with extra layers of sweat and subtle details indicating his new life in Sakaar."

"Performance wise," adds ILM animation supervisor Steven Nichols, "ILM had the exciting opportunity to have Hulk carry out full-blown conversations! Mark Ruffalo's performance was captured on set to maintain the timing, dialogue and relationship with the other actors. Due to the improvised nature of some of the scenes, this approach allowed a level of flexibility to capture new ideas or different takes that really made Hulk's character emerge through Chris and Mark's exploration."

Video of Thor: Ragnarok - 'We Know Each Other'

Morrison gives ILM much credit for delivering this new kind of Hulk for the Sakaar scenes. He also, of course, says that Mark Ruffalo was easily able to adapt to provide the definitive Hulk performance.

"Mark is the Hulk, he's the best at being Hulk in the whole world. Every time he comes to doing Hulk, he gets back in the character and I guess it's like riding a bike. But it was fun to see a fundamentally different Hulk this time around."