Wow, that sex must have really been great. It sounds like it was great, in the shrouded opening moments of “Frankie and Johnny in the Clair de Lune.” And as the afterglow and the moans give way to the harsh lights of a Hell’s Kitchen apartment, playwright Terrence McNally makes clear that this evening has rocked Johnny’s world. He’s convinced this first date is more than just that. It’s a moment begging to be seized.

Casting Spells Productions has mounted this 1987 two-hander with Charles Hubbell and Shanan Custer rummaging through the emotional weirdness of an alpha dog on the scent and a resistant quarry who fends off his ardor. Director James Detmar’s production, which opened Thursday at the Minneapolis Theatre Garage, felt timid and sluggish; we miss the wave of cheer that should wash us with good wishes for these ordinary chumps.

Hubbell’s Johnny is a humble peacock — strutting even as he acknowledges his flaws. He is street-smart, wise in a way, yet here he is at 46 years old working as a short-order cook. And he knows that. He is keenly aware of life’s ticking clock, his failures and the few opportunities to capture at least a little happiness before his world goes to dust.

“People are given one moment to connect,” he says in one of the play’s most famous lines. “They don’t take it, it’s gone forever.”

Various shades of truth animate that idea, but Johnny’s New Yorkese makes it sound like a sales pitch — and Frankie isn’t buying.

Custer struggles with this role, which is more complicated than Johnny’s full-frontal assault. Detmar, who played Johnny in a beautiful 1988 Cricket Theater production, cannot find places for Custer to go beyond the embarrassed smile and nervous laughter. She works hard and courageously, but perhaps she is working too hard rather than internalizing Frankie’s emotional scars and exploring the naturalism of McNally’s script.

There is fire in this production, but it rarely generates heat. Is there an unseen dream these two are dancing around? Or are they merely fumbling in the dark? Key moments — when Frankie’s iron resistance starts to crack and later when regret grips Johnny — are not sharply observed.

McNally’s second act seems a repetitious continuation of the “greatest-night-of-sex-in-history-and-the-resulting-pursuit-of-more” motif that Frankie and Johnny have fallen into. But that is the play he wrote, so that is the play we watch.

This light romantic comedy cannot bear a lot of psychic weight, nor should it. It does, however, require an emotional investment that has us humming Debussy’s great “Clair de Lune” and wishing the best to Frankie and Johnny. This production accomplishes only the former.