The only moment on Monday night when the mezzo-soprano Anne Sofie von Otter sounded like an opera singer was during her second encore, a song by former Abba member Benny Andersson called “Butterfly Wings.” In the context of a soft, sweet, poplike song with its simple accompaniment, her voice suddenly sounded big and rich and sophisticated and a little overblown — the way classical singers tend to sound when they’re doing so-called crossover.

It was a striking contrast. For the rest of the evening, she sounded simply like a person singing to express herself, giving such a realistic impression of artlessness that it wasn’t until this encore, at the very end of the evening, that she lifted the edge of the curtain to show that there was, after all, a little more to the exercise.

There’s loads of artistry behind von Otter’s delivery, of course. She couldn’t have sustained the major career she’s had, or still be singing with girlish freshness at 57, without it. But the secret to her success, I believe, is that she’s like the singer next door; she seems as though she just happens to be sharing with you a couple of songs she likes.

When she launched into Charles Trenet’s “Boum” for her first encore, hamming it up and making faces to go along with the sound effects, she was less a diva than your friend’s kid sister, jazzed and a little carried away by her ability to make you laugh. When she sang “De Vilda Svanarna” (“The Wild Swans”) by Sigurd von Koch, she made the fullness of her voice seem like a natural outgrowth of the song. Von Otter doesn’t do vocal climaxes; she does words, and the singing follows with deceptive ease — or with true vulnerability, as when, at the end of Reynaldo Hahn’s “L’Heure Exquise,” she raised an eggshell of a note made poignant not because it cracked, but because one was aware, hearing its thin arc, that it so easily might: a fragile thing to guard in your cupped and listening ear.

Hers is not a big voice. At this stage of the game, it’s slightly dry around the edges, like an old Sauternes with rich sweet notes and patches of seductive heaviness that’s just beginning to lose some of its flavor. Combine this and the knack for spoken delivery, and you get a great voice for songs, particularly songs with sustained lines and thoughts, such as Grieg’s ­“Varen” (“Spring”), though perhaps less musical effectiveness in quick songs such as Sibelius’s “Vilse” (“Astray”), which sometimes carried her off the notes and even the rhythm altogether.

Mezzo-soprano Anne Sofie von Otter (Ewa-Marie Rundquist)

Officially presented by the Fortas Chamber Music Concerts, the program was also part of “Nordic Cool,” the ongoing Kennedy Center festival of all things Scandinavian, and it obligingly offered a first half devoted to Scandinavian composers — although no Danes. Von Otter observed that Sweden didn’t have titanic musical figures such as ­Grieg or Sibelius while offering a couple of tastes from what she described as a smorgasbord of song-inclined Swedish composers — a heroic, drawing-room-style ode called “Nothing Is Like the Time of Waiting” by Wilhelm ­Peterson-Berger, or Wilhelm Stenhammar’s folksy “Old Dutchman.”

The second half was all French and, if anything, even better. The great thing about von Otter’s French singing is that she doesn’t set out to “do” French style; she simply inhabits it. Fresh naturalness suits Debussy’s “Chansons de Bilitis” oh so much better than the heavy-lidded faux-archaism with which many singers, even unconsciously, invest it; these songs have to be at once sexy and sexless, and von Otter is just the singer to pull that off. Ditto the fine set of Rey­naldo Hahn that followed the intermission; von Otter first invoked Hahn’s dandyism and languor, and then stripped his music of it. And although she could get a bit too rustic and hey-ho, with lustily swaying elbows, in folk-inspired songs such as “Old Dutchman,” she opened her throat and dropped the stereotypes in “Bailero,” the first of four of Canteloube’s “Chants d’Auvergne.”

Bengt Forsberg has been accompanying von Otter for years, and he got to play up the accompanist’s tacit straight-man role, drawing laughs with his dry ripostes to some of von Otter’s comments. He also took the mike a couple of times to give equally natural introductions to a few solo piano works; he said that Chabrier was an old friend of his in such a natural and heartfelt tone that you thought he meant it literally (Chabrier died in 1894). And his performance matched von Otter’s to a T; sometimes a little approximate in terms of technical polish, but full of openness and honesty, with an additional emphasis that the singer didn’t always muster.

Von Otter is singing with the National Symphony Orchestra on Thursday, Friday and Saturday. You should probably try to hear her.