Rachel Bay Jones in her dressing room at the Music Box Theatre in New York. She won a Tony Award in June for the musical “Dear Evan Hansen.” (Jesse Dittmar/For The Washington Post)

NEW YORK — Rachel Bay Jones has a theory about the roots of the terrible act upon which everything else hinges in the Tony Award-winning “Dear Evan Hansen” — the lie that Evan perpetuates and that ultimately causes such enormous heartache. Her hypothesis concerns not so much the grieving family he hurts, but rather the rift in his own family, the drifting apart of Evan and the character Jones plays, Evan’s mom, Heidi.

As she sees it, the key to the show’s devastating turn of events is the fraying of the bond between Evan, a teenager of profound anxieties, and Heidi, an overstressed single mother. At a time when parent and child normally separate, these two don’t, not healthily and not in equal measure. They’re so accustomed to being there for each other, she says, that when the distractions multiply, with Heidi  overwhelmed by other responsibilities  and Evan growing ever more socially isolated, the son l ooks elsewhere to restore his sense of stability.

“When that relationship is torn apart for whatever reason, and the closeness is lost, as naturally happens to teenagers, he goes to another family for that,” she says. “It explains so much about why Evan could lie so easily to this other family, because he was used to taking care of his single mom.”

Jones has had lots of time to ponder the psychological framework of “Dear Evan Hansen.” She’s been with the show for more than three years now, having played Heidi since the first reading in Manhattan in 2014. She was in the versions of the show that premiered at Arena Stage in Washington in 2015 and then off-Broadway at Second Stage Theatre, before it moved to Broadway last fall. She and Ben Platt, who plays Evan, have spent these years forging one of the more nuanced mother-son connections the musical theater has known, a sophisticated collaboration the theater world enshrined last month by voting them both Tony Awards for their performances.

Ben Platt and Rachel Bay Jones in "Dear Evan Hansen." (Matthew Murphy)

Platt has been widely celebrated for his wrenchingly intense, all but superhuman exertions in the musical by songwriters Benj Pasek and Justin Paul and book writer Steven Levenson. But the 47-year-old Jones — who grew up in South Florida and has been working in the theater since arriving in New York in 1989 and almost instantaneously being cast in a Broadway version of “Meet Me in St. Louis” — creates in Heidi something just as remarkable. That fact is affirmed in powerhouse moments such as the evening’s penultimate number, the ravishing “So Big/So Small,” about the breakup of her marriage and its impact on Evan.

In some ways, her challenge is even more formidable, given the time constraints of bringing supporting characters fully into being in a musical. Jones has to meet the demands of animating Heidi’s vulnerabilities and strengths in highly economical fashion. In an interview recently in the green room backstage at the Music Box Theatre, she talked wistfully about the scenes that had to be trimmed and lines that were cut that more luxuriously explored the boundaries of Heidi and Evan’s lives together. Fortunately, three years in the skin of another person provide an actor with a useful cushion, especially as that process pertains to a self-described “slow, deep learner” such as Jones.

“I know this woman,” she says of Heidi. “She’s real. And she’s not me. She’s not. There’s a lot of me in her because what else can we bring out, but our own experience and our own observations? 

“I have a 14-year-old daughter, so a lot of this comes from personal experience,” she adds. “A lot of this comes from the relationship I have with my own mother, the relationship she has with her mother. And with Heidi in particular, something I can definitely relate to is that she’s just trying to survive, you know? There’s an instinct that parents have for survival that teenagers don’t sometimes have. We know this about life. We know that when something comes up, it’s like, ‘Buck up! Get your s--- together! It’s a long road!’”

Rachel Bay Jones in New York. (Jesse Dittmar/FTWP)

The road for Jones began in Boca Raton, Fla., in a household filled to the rafters with drama: Her parents, Dennis Jones and Mona Feit, classical actors who worked extensively at Shakespeare festivals, were, as she describes it, “very grand.” Her father was “this kind of reserved, composed guy, and my mother is the most explosive, the biggest, the most everything — and as a parent she’s like that, too. Shakespeare was freely quoted in the house and that was just the way they spoke.”

They left the business after Rachel and her brother came along, opening up health food stores (and eventually retiring to Hawaii). Their introverted daughter didn’t grow up wanting to emulate them. But at some point in adolescence, she succumbed, and dropped out of Spanish River High School in Boca to appear in a production of William Inge’s “Picnic” at Florida Atlantic University, where a cast mate was Marc Kudisch, an actor who would become a lifelong friend.

Along with a group of students from the university, she moved to New York, lacking any formal training in acting or voice — to this day, she says, she’s never taken a class. “I’ve never been a performer performer, like, ‘Look at me! Look what I can do!’ I can’t function on that level. As soon as I start to try doing that, I crumble.

“So,” she adds, “I don’t really want to be looked at. But I desperately want to be seen.”

Jones’s stage career is one of those that has accelerated as she has matured. After appearances in the Broadway revival of “Hair!” in 2009 and a musical adaptation of the movie “Women on the Verge of a Nervous Breakdown” in 2010, a bona fide breakthrough occurred for her in the 2013 revival of “Pippin,” a Tony-winning revival in which she played Catherine, who sings the buoyant Stephen Schwartz song, “Ordinary Woman.”

And then came “Evan.” Levenson, among others, has observed that right from that initial reading, Jones’s portrayal became a part of the project that the creative team wanted to preserve. That’s great for an actor’s confidence, but it hasn’t meant that living inside a musical that deals with such sensitive topics as suicide and mental illness has become any easier. “It’s hard to want to go to work some days, because it’s so deep and it requires so much of every cell,” she says. “You just wake up some mornings, and your chest hurts.”

You don’t tend to think of being in a musical as something to recover from. That’s why for Jones, the hopeful final scene is a particular blessing.

“There’s a gentle uplift, a beautiful sort of flowering that comes out at the end of the show, and that allows all of us to walk away without wanting to end it all,” she says, laughing. “Heidi needs that, and Evan needs that. And Rachel needs that.”

Dear Evan Hansen, music and lyrics by Benj Pasek and Justin Paul, book by Steven Levenson. Directed by Michael Greif. Tickets: $119-$499. At the Music Box Theatre, 239 W. 45th St., New York. Visit telecharge.com or call 212-239-6200.