Why Prey's mud pit scene was so tricky to film

gamesradar.com:

Prey star Amber Midthunder was put through the wringer while filming the Predator prequel, from learning to fling around a tomahawk and using a bow and arrow to working with a super hyperactive dog. But there was one particular scene – first glimpsed in the official trailer, and involving a whole lot of "mud" – that she found particularly challenging. 

The sequence sees her character Naru, a young Comanche warrior who goes rogue when her tribe refuses to believe she's seen a near-invisible force skinning snakes and butchering bears near their camp, try to elude the menacing alien. To do so, she submerges herself in a mossy sludge, which shields her from the Predator's heat signature technology. It took nearly a whole week to shoot.

"We did it right towards the end and we'd had lions and bears, oh my, and all that stuff and then we get to the mud pit," Midthunder tells Total Film. "We would do splits, we would come in in the early evening when the sun was still up and we'd do that for the first half of the day. I would get hosed off at lunch and then go film fight scenes at night. 

"I had heard about it beforehand," she laughs. "Somebody had done a test and was like, 'Oh, it's not that bad, it's just like water with leaves in it.' So I was like, 'Easy, great, amazing, it'll just be like a mud bath or whatever.' Then I get there, and first of all, I don't even know what it was made of. I don't want to know. It was thick, I feel like it sunk into my skin and probably is still with me somehow. It's still in my blood a little bit. It had this smell that Dan [Trachtenberg, the director] told me recently, he said, 'It didn't smell that way until it touched human flesh.' Like, the most disgusting thing to say. He said as soon as