Learning to take and throw a punch is one thing, but changing your whole physique to better emulate one of the most awarded athletes in the sport of boxing is a different task entirely. For Ryan Destiny, originally an R&b singer turned actress on shows like “Star” and “Grown-ish,” becoming two-time Olympic gold medalist Claressa “T-Rex” Shields in Rachel Morrison’s “The Fire Inside” was unlike any challenge she’d faced before.

“It was a lot. It was really, really exhausting, but in a good way,” Destiny said to the Los Angeles Times in a recent interview. “Because I have never pushed myself that way ever in my life before. Especially not physically. So to see my body transform from day one until actually starting was so crazy to me because I didn’t know my body could do those things that it was doing. And I built my...