Ailyn Pérez, left, took the stage solo when her husband, Stephen Costello, fell ill. (Josep Molina/Molina Visuals)

Stephen Costello and Ailyn Pérez are rising opera singers who are trying to break into Love Couple territory. That is, they’re a tenor and soprano who play lovers onstage (for example, in the Washington National Opera’s “Elixir of Love” earlier this year) and are married in real life, and they’re trying to parlay this into a career steppingstone. They’re currently touring with a duo recital program to help promote a recent CD release, and WNO brought them to town to open the company’s season at the Terrace Theater on Wednesday night.

That, at least, was the plan. But plans don’t always go the way they’re supposed to. When the pair took the stage, Costello announced that he was under the weather but was going to try his best; a few phrases into the planned opening duet from “La traviata,” as he was heading for a high note (and sounding, incidentally, pretty darn good), he turned to the pianist, Danielle Orlando, and said, “I’m done.” The artists scrambled offstage, the audience buzzed and, after a few moments, Michael Mael, WNO’s executive director, took the stage and rather ungallantly apologized for the fact that Pérez would be shouldering the evening herself, conveying the idea that he hoped the audience wouldn’t mind too much but would understand if people left — hardly a flattering way to present a solo singer faced with offering a one-woman performance she hadn’t planned on giving.

From my vantage point, I saw only three people take advantage of Mael’s offer. The rest stayed put. The fact is that, while Costello and Pérez both have burgeoning international careers, they are not yet big enough stars that audiences ache with disappointment not to see one or the other. Pérez stepped quite happily into the spotlight with an energetic recital, buoyed by the audience’s sympathy for her challenge of pulling together a solo program on the fly, and won herself a lot of fans in D.C. as a result.

Pérez has an attractive and capable voice. Like many singers today, she has a slight tendency to paint by numbers: She knows how to offer a particular arsenal of effects and deploys them appropriately as called for. Here, you get a melting top note; here, operatic fire; here, wide-eyed coquettishness; and at the end of most big numbers, a big note and an upraised arm.

Well and good, and it’s arguably unfair to call her out for a lack of variety in a program that she wasn’t planning on giving. Note, though, that I found her performance of De Falla’s “Siete canciones populares Españolas” (seven Spanish folk songs), a repertory staple that was on the scheduled program, anodyne, while some of her last-minute additions, including Desdemona’s “Willow Song” from Verdi’s “Otello” and “Ch’il bel sogno di Doretta” from Puccini’s “La Rondine,” were highlights of the night.

Orlando’s somewhat heavy playing, and the piano’s open top, did Pérez no favors in the song repertory. (She also did three songs by Obradors, unscheduled, and three songs by Reynaldo Hahn that were on the printed program.) When she sang opera, however — including Juliette’s waltz from Gounod’s “Roméo et Juliette” and Mimi’s first aria from Puccini’s “La Bohème” — she shifted into a higher gear that enabled her to be heard better. (This was even true in Gershwin’s “Summertime,” which is not necessarily a high-gear piece.)

Pérez seems to have figured out a formula that works well for her: She’s attractive, makes a nice sound and can deliver the goods. She backed it up with a kind of charming maladroitness in her public remarks. (“He’s the talker, and he’s the funny one,” she said into the mike of her absent husband, “so give me a break.”) I found a certain emotional vacancy or sameness to the evening, but you could blame the situation for that, and the enthusiastic audience was certainly delighted with the performance.

The evening’s intended message was certainly subverted, in a way that perhaps illustrates the pitfalls for a young couple of turning their relationship into a professional commodity. Some ambiguity already emerged on the printed program; Costello had planned to sing three Tosti songs that included “Non t’amo piu” (I don’t love you anymore) and “Goodbye.” Pérez, meanwhile, telegraphed the message that she could do just fine on her own, concluding with a fierce “Love is Where You Find It” — by now, she had kicked off her high-heeled shoes — with lyrics stating that love is all around and that an old love is soon replaced with another. No one will read too much into this; for the audience that WNO seems so eagerly to be courting (Pérez, too, expressed an almost abject gratitude to everyone for coming), it’s just part of the show.

WNO’s season opens on Sept. 20 with “Florencia in the Amazon.”