The white jackets of the musicians seemed the perfect background for the rather laid-back leadership style of guest conductor Christian Zacharias as the Mostly Mozart Festival Orchestra presented their latest collection of Wolfganglia Friday evening. In a maneuver evocative of 18th-century musical logistics, Mr. Zacharias began and ended the concert with the Posthorn Serenade, K. 320, commencing the first four movements at eight o’clock and saving the last three until a few minutes before 10 p.m.

After some initial messiness, the orchestra offered a relaxed but decidedly unfocused rendition of this extended piece of tafelmusik. There was a nice bounce to the rather slow pace, reminding me of music to accompany a skate at Rockefeller Center, even though the seasonality wasn’t right. Were this indeed a winter concert, I might have pronounced the not very crisp attacks and accents as flabby, but as this was summer, let’s just label this performance as pleasingly plump.

One reason for the immense popularity of Mozart in our time and the relative unimportance of Haydn’s corresponding music — the reverse of 18th-century sensibilities — is that Mozart generously endowed his art with large quantities of angst much more relevant to the modern age than the deist optimism of his Viennese mentor. The most powerful music of the boy genius always contains the elements, if not the expressed reality, of sorrow and dissatisfaction.

Even the comic operas contain many examples of the deepest melancholy (“Oh, darkest night” from “The Magic Flute”),fear (expressed most deliciously in Soave sia il vento from “Cosi fan tutte”), and confusion (Non so piu cosa son from “Figaro”).And there are some works that are pure 20th-century existential despair, such as the Adagio and Fugue in C minor and the first movement of the amazing Piano Concerto #20, which this evening Mr. Zacharias conducted from the keyboard.

Well, at least he waved his arms around when he was not playing, although how much his nebulous directions influenced his charges is open to debate. As a pianist, Mr. Zacharias adopted that same lackadaisical style that he had demonstrated in the serenade, dampening the darkness of the opening Allegro and eliminating a good portion of its drama.He is a solid technician with a remarkably uniform touch — clean and steady if a bit monochromatic. The left and the right hand were balanced like the scales of justice, but this was questionable interpretively, as the left never intoned the more ominous notes in a suitably strident manner. In fact, Mr. Zacharias seemed to come to life only when performing his own rather Lisztian cadenzas. Left essentially to its own devices, the orchestra simply seemed to go along for the ride.

Also on the program was a contemporary take on all things Amadeus, Dieter Schnebel’s Mozart-Moment from 1989, a clever, aphoristic piece that sounded a bit like Mozart’s Music for a Musical Clock, with the springs wound much too tightly.

At intermission, I moved my seat to the very back of the hall as I often do to evaluate singers. Sally Matthews is a young Brit who has sung on several occasions for Sir Simon Rattle. She presented two concert arias, one very familiar and one relatively obscure.

The concert aria is a lost art form that peaked with Beethoven’s “Ah! perfido.” Not exactly a song with orchestra nor an opera highlight, it is rather a heroic utterance, often invoking the ancients, a wayfaring scene searching in vain for a production.

Ms. Matthews was quite impressive. She certainly projected volume to reach the farthest points in the house (there is, though, a parabolic effect at Avery Fisher Hall that assists singers in this regard). And she invested her accounts of the powerful “Bella mia fiamma … Resta, o cara” and the more tender “Vado, ma dove?” with a committed emotional range and a believable character portrayal. Technically, her individual note dynamics and mini crescendos that added considerable color proved extremely pleasing.Tones were warm and fully formed. Even though there were occasional sharp moments in run-ups to climactic high notes, the ultimate pronouncements were uniformly and thrillingly on pitch. Thankfully, Ms. Matthews injected some genuine human passion into an otherwise rather phlegmatic evening.

Until August 26 (Lincoln Center, 212-721-6500).