Jamie Campbell Bower was 'terrifed' of doing THIS in 'Stranger Things' season 5

Jamie Campbell Bower was "terrified" to play Mr. Whatsit in Stranger Things season 5.

In Season 5 of the show, Mike and Nancy Wheeler’s younger sister Holly takes center stage. Mr. Whatsit enters the show as a figure Holly keeps seeing and talking to.

"It was a really fun experience, but it was also one that I was quite scared of as well, because I didn't necessarily see Whatsit in Henry," he told People.

"They're not really the same, so it definitely felt like I was building something new," he added of Mr. Whatsit who earns children’s trust through his kind and loving manner.

The new season follows Henry’s pursuit of Holly and her classmates, 12 of whom he successfully kidnaps and takes to the upside down.

This meant Bower worked with frequently with children this season.

"Children see through the bull----. They know," he said. "They just know when you're being real with them. That's really scary to sit there in front of somebody who's like, 'Are you lying to me?'"

In another interview with Deadline, the actor revealed that creators Matt and Ross Duffer developed Mr. Whatsit after season 4 ended.

"Towards the end of filming the Henry portion of season four, Matt and Ross [Duffer] were like, 'This has been amazing. We were always going to bring you back as Vecna, but now we feel like we should bring you back more as you.' and I was just like, 'Cool. Sounds great. Whatever you guys want,'" Bower recalled.

"So, with regards to the story arc, none of it was pitched to me, and I think it was … something that sort of developed in the in-between time," he added.

Stranger Things season 5 volumes 1 and 2 are streaming now on Netflix. The finale drops on New Year’s eve.