Until this week, I wasn’t sure why I had waited until the end of the run of Falsettos – it closes on Sunday – to write about Christian Borle and Andrew Rannells. What was it about their performances as Marvin and Whizzer that had moved me during the course of the engagement -- from the invited dress rehearsal to a late-preview performance to a student matinee to the moment, earlier this week, when I watched the show for a final time as it was being filmed for a future airing on PBS’s “Live from Lincoln Center”?

Was it the wary, early stages of their courtship, in which they couldn’t agree whether they’d been together nine months or ten? Was it the plaintive tones each offered in the haunting, ambivalent act-one quintet “I Never Wanted to Love You”? Was it their subtext of frustration in their later duet, “What More Can I Say” or the Weimar-cabaret bitterness with which Whizzer seems to cast off both life and his love for Marvin in “You Gotta Die Sometime”?

Yes, yes, and yes. And, yet: no.  What stayed with me all these months was much more basic: I believed that these actors, in Falsettos, were a couple. This would seem so fundamental a requirement for any love story as to require no comment. But all-too-often a credible bond goes missing in a play or movie, even when – especially when – it would seem a given. Watch, for example Brad and Angelina in the 2015 picture By the Sea, in which their attempt to repair a marriage makes no sense because you never believe they had much of an attraction in the first place. Or, apart from Who’s Afraid of Virginia Woolf?, view virtually any of the 11 movies that Elizabeth Taylor made with Richard Burton. Their onscreen sparks rarely conflagrated as wildly as those they stoked in real life. As for theatrical examples, I can think of several in which the parties were true pals backstage but, once they swept onstage, turned ice-cold.

Who can explain it, this phenomenon? Jon Robin Baitz did, in his LCT-produced play Other Desert Cities. “Life is chemical,” says one of the characters. So are on-stage partnerships. And appearing before the public can provide an ion-charged atmosphere in which two actors can ignite, regardless of their rapport out of the spotlight.

Borle and Rannells, I can assure you, have been comfortable hanging out together backstage. In Borle’s dressing room, they were often to be found sitting on the sofa, shooting the breeze. Their comfortably chill backstage demeanor, however, never quite prepared me for the emotions they generated onstage. At every performance, I was unglued especially by Marvin’s solicitousness when, in a game of squash, Whizzer collapses on the court, assuming, almost, the pose of that ultimate wounded-warrior sculpture of antiquity, The Dying Gaul. And I was invariably touched by the affecting emotional interstices in the lyrics of their number “What More Can I Say.”

What most, however, convinced me that these characters, in these performances, love each other is the finale, “What Would I Do?” I always think that I’m prepared for Marvin singing: “Once I was told/That good men get better with age.” But then he follows up with “We’re just gonna skip that stage.” And that’s it: the tap is turned on and I’m gone. I wish I could say that, in a lifetime of theatergoing, I remain as emotionally available to heart-catching moments as I was when, as a young man, I sat in the cheap seats. But I confess that I have hardened with age: I’m too aware of the manipulative tricks of the performer.

Thank you, Christian Borle and Andrew Rannells, for piercing my armor.

Brendan Lemon is the editor of lemonwade.com