Mark Ruffalo wasn't available to film all of his scenes in Thor: Ragnarok, which led to director Taika Waititi stepping in to film some of the motion capture scenes in his stead. This is in addition to his previously-revealed mo-cap/voice work as Korg and Surtur.
"I’m playing one of the 'Planet Hulk' characters: Korg, who’s a Kronan and a bit bigger than me. It’s all mo-cap. And often, since Mark [Ruffalo’s] no longer here, I’ll often jump in for Hulk’s stuff," Waititi told Collider. "We have stand-ins, but they’re not actors and they don’t have timing so I’ll jump in for those things."
Waititi said that he might have his Surtur role dubbed over by another voice actor.
Although best known now as a director thanks to his Thor: Ragnarok job, Waititi has acted in numerous films including Warner Bros.'s Green Lantern.