The actress portrays Hunter B-15, a member of the Time Variance Authority, in the new Disney+ drama series, a spin-off from the Marvel Cinematic Universe, and she is tasked with keeping Hiddleston's titular Norse God of Mischief in line.
However, whenever it came to filming the fight scenes, Mosaku found herself always checking on Hiddleston to ensure she didn't hurt him, because she's haunted by an old incident from her drama school days.
"At drama school I had a moment where I actually punched one of my best friends during our stunt training class, and so I've never really gotten over the guilt of that, and I'm quite clumsy as well, so I was really kind of nervous that I would injure our lead!" she confessed on "Good Morning America".
Hiddleston's on-camera responses to the blows also did little to put Mosaku at ease, even though it was all part of his act.
She added, "He's really good at selling all the action stuff, so you hit him..., and it sounds worse than it is, but he would react really big, and I'd be like, 'Oh my gosh, did I hurt you? Are you OK?' and he was like, 'I'm good, just keep going!' "
Meanwhile, Mosaku admits landing a role in the MCU was a dream come true for the star, even though she had no idea she had actually tried out for a role in the superhero franchise.
"We're so used to auditioning for top secret things and sending it out to the ether, so I had no idea that I was auditioning for the MCU," she shared, admitting it was actually a blessing in disguise.
"Good job, 'cause I probably would have messed it up 'cause I would have been really nervous!"
Recalling the moment she learned she had landed the coveted gig, she said, "I was at my mum's house in England and I got a phone call from my agents in the U.S. [in the middle of the night], saying I got offered Loki, and I was like, 'When did I audition for Loki?' "
"I was like, but yes, absolutely! [I'm] a huge fan of the MCU so I was like, 'Whatever it is, I'm down!' "
"Loki" debuted on the Disney+ streaming service on Wednesday, June 9.