In Guess How Much I Love You, Sheehy stars opposite Bafta Best Actor winner Rob Aramayo as a couple expecting their first child, and receive devastating news at their 20-week scan.
Performing eight shows a week, the actress says such an emotional performance “does take it’s toll”.
“You’re telling it as as truthfully as possible and giving it the story that the credit and the weight that it’s due and that’s a responsibility which I really don’t take lightly,” she said.
“The older I get, the more I kind of take that kind of care more seriously, I think, that you have to look after myself, you know, and my own mind, body, and soul.”
Her critically acclaimed performance is what earned Sheehy her second Olivier nomination, something she’s in “a state of shock” at.
“I really, really didn’t think I’d ever be nominated again in my life.
“Last year I was kind of like, OK, we’ve done that though, we’ll do that.
“So to be nominated again was really mind-blowing”.
The actress said she performed as “organically me” with her Port Talbot accent.