Ms. Spencer-Jones said that the training is not just a matter of showing off a fit cast.

“Physique is second to me, really,” she said. “Alex makes a point in the book of saying as a young man, you should be the best version of yourself that you can be. And certainly Jonno’s translation of that is: ‘Don’t let your body go to waste when it’s in its prime.’”

The play’s vividly physical displays of sexuality and violence (shirts? what shirts?) are complemented by a contemporary soundtrack that includes David Bowie, Muse and, in a nod to the novel, modern-day versions of lovely Ludwig Van (Beethoven). It’s not a musical, but there are complicated, intense, erotically charged choreographed fight scenes that require the cast to be in top shape to get through the show night after night.