The first thing you notice about Stephen Costello, in his return to the Metropolitan Opera, is how tall he seems.

The tenor from Northeast Philadelphia seems to have grown by six inches. As with most opera stars, all explanations are rooted in The Voice. “Anybody who has confidence walks really tall, and I feel like that when I walk onstage now,” he said. ”I don’t have to worry about my voice. … It’s going to be there.”

The posture is also evidence of a revised breathing technique that requires him to stand straighter.

Costello has just broken a season-long absence from the Met with an eight-performance run of La Traviata ending April 27. On May 10, he steps into Rigoletto. Next season is Maria Stuarda with Diana Damrau. Future seasons in other houses include Don José in Carmen, his heaviest role so far.

Anita Hartig as Violetta and Stephen Costello as Alfredo in "La Traviata" at the Metropolitan Opera.
Ken Howard / Metropolitan Opera
Anita Hartig as Violetta and Stephen Costello as Alfredo in "La Traviata" at the Metropolitan Opera.

Though his career seemed to be business as usual during 18 months or so away from the Met, Costello, 37, was reworking his voice, making changes that aren’t easy in midcareer. It’s why he was able to hammer out the notorious nine high C’s in the Daughter of the Regiment aria “Ah! mes amis, quel jour de fête!” on a new Delos-label recording titled A Te, O Cara: Stephen Costello sings bel canto.

Met orchestra members heard the improvements. Most of the flowers tossed his way during La Traviata curtain calls this month have come from the instrumentalists.

“They’ve played for you all these years … and have noticed a difference,” Met orchestra violinist Yoon Kwon Costello, his wife of nearly two years, told him recently. “Even when we had no relationship, I used to get nervous for you.”

Evidence of this new security was in the apparent ease of his last-minute replacement for Juan Diego Flórez earlier in the Met’s Traviata run. Costello was still jetlagged from an engagement in Dresden and was out Christmas shopping when the Met called on him to sing Dec. 26.

It killed his holiday plans in Philadelphia with his family, but he was reunited with Met music director Yannick Nézet-Séguin, with whom he hadn’t worked since a 2010 Salzburg run of Romeo et Juliette.

Yannick Nezet-Sequin, rehearsing the Metropolitan Opera.
KenHoward / MetropolitanOpera
Yannick Nezet-Sequin, rehearsing the Metropolitan Opera.

There was no time for rehearsal. “He came to my dressing room before each act and said, ‘This is what I’ve been doing here, but I can change it.’ But he didn’t have to change anything because everything he does makes you feel comfortable,” said Costello. “It really was about making music together.”

The native Philadelphian, who grew up around the family-owned Jack Costello Boxing Club, had been a promising trumpet player (which kept him out of the boxing ring to protect his lips), sang in the chorus at George Washington High School, studied at University of the Arts, and then truly developed his voice at Academy of Vocal Arts.

He’s one of that Philadelphia conservatory’s great success stories, having gone straight from the Spruce Street headquarters to the 2007 Met production of Lucia di Lammermoor.

Stephen Costello in 2007's "Lucia di Lammermoor."
KEN HOWARD / Metropolitan Opera
Stephen Costello in 2007's "Lucia di Lammermoor."

He and his then-wife, soprano Ailyn Pérez, whom he met during their AVA years and married in 2008, even had “his and her" Richard Tucker Awards for promising opera stars of the future, his in 2009 and hers in 2012.

Yet not long after they were touted as the new love-couple of opera, the marriage unraveled. ”It is with great sadness that I share the news that my wife, Ailyn Pérez, and I have separated, with the intention of divorcing,” read his Facebook message in early 2015, only months after they had released their Love Duets album. Neither of them publicly discusses the details of the break.

Costello then did what many newly divorced guys do: He moved back in with his parents, who live in the Philadelphia Mills area of the Northeast. An interview with the Daily Beast carried the sensational headline “Heartbreak almost destroyed this opera singer’s voice,” referring to a single Met cancellation when he suffered a neck spasm – yes, due to stress – that temporarily impaired his voice.

Despite their breakup, Costello and Pérez honored a long-standing Santa Fe Opera commitment in 2016 to sing Romeo et Juliette, by all reports a harmonious and professional effort. But even though their repertoires intersect nicely, they won’t be heard together in the future. “It’s not so much us as the opera houses,” Costello said. “Even if there isn’t a conflict, they try to avoid it.”

Meanwhile, Costello’s midweight lyric tenor had never been as secure as he — and those around him — would have liked. High notes were iffy.

A problematic tonsillectomy yielded some improvements. But Costello had a sense that he and his esteemed vocal coach Bill Schuman, who had cultivated the Costello voice since 2003, had gone as far as they could.

Revising his vocal technique with Anthony Manoli, whom he knew from his work with soprano Sondra Radvanovsky, was productive but nerve-racking.

“It’s like being a child actor," Costello said. "You become fashionable or visible at such an early age … and you’re less willing to change anything in the equation for fear that you’ll throw it off. But there was so much I wanted to do vocally and couldn’t find a way to translate it. … I was lacking tools.”

Stephen Costello as Alfredo and Placido Domingo as Germont in this season's "La Traviata" at the Metropolitan Opera
Ken Howard / Metropolitan Opera
Stephen Costello as Alfredo and Placido Domingo as Germont in this season's "La Traviata" at the Metropolitan Opera

Though lyric tenors of his caliber enjoy special status because good ones are so rare, Costello knows that he’s on a tightrope like everybody else. “In opera you can be on top of the world one day and a year later be at the bottom of it,” he said, “and if it gets to the point where you’re concerned every time you go onstage, they aren’t going to want you onstage. Look at the Met and how many productions they do every year. They want to hire people they can count on.”

He also has a Northeast Philly work ethic, coming from a family where his dad’s arrival home from work meant that mom was off to do the evening shift as a supermarket cashier.

Vocal coach Manoli was impressed how quickly and quietly he shored up his vocal foundation while maintaining a full performance schedule. “The fight-or-flight tendency is to go back to the old way of doing things,” Manoli said. “The calmness with which he did the work was very admirable."

Taking control of his own voice coincided with his budding relationship with Kwon. They were married at New York’s City Hall on June 30, 2017, three months after they began dating seriously — with neither family present.

“Since we had both been married before, we know that big weddings aren’t for the couple but for everybody else,” said Costello. “We took pictures. We had tacos. Then we got in the car and drove to Philadelphia to pick up the engagement ring that I had bought for her.

"I accidentally told my parents the next day. I was talking about changing the car insurance and my mom said, ‘Why do you have to do that?’ ”

Stephen and Yoon Kwon Costello on their wedding day
Courtesy Stephan Costello
Stephen and Yoon Kwon Costello on their wedding day

In his apartment on the 12th floor of a newish Manhattan apartment complex — where Bruce Willis is a neighbor — Costello seems a long way from his roots.

Professionally, he is in an odd in-between zone that promises no immediate return to Philadelphia concert halls. Opera Philadelphia is invested in new repertoire. The Philadelphia Orchestra has only started performing the sort of operas in concert for which Costello would be appropriate.

But Philadelphia — in the form of his extended family — comes to him.

“For them, coming here … isn’t just getting in the car or on a train. They rented a limousine, got 15 people in it, and had a party all the way to our apartment,” Costello says. “It gets super loud in here. We’re from Philadelphia, so we always yell when we’re in a big group.”

Stephen Costello sings Rigoletto at the Metropolitan Opera on May 10. Information: metopera.org or 212-362-6000.