The film director set himself a steep challenge in his debut stage work. At least for now, he hasn’t quite met it.LISBON — Gus Van Sant is no stranger to experimental biopics: “Last Days,” his lyrical, nearly dialogue-free meditation on the end of Kurt Cobain’s life, shunned every convention of the genre. Yet “Andy,” his Andy Warhol-inspired stage debut, which had its world premiere at the Teatro Nacional D. Maria II in Lisbon this week, may be Van Sant’s oddest tribute to date.For starters, it’s a musical. Warhol duets with the modernist art critic Clement Greenberg; Valerie Solanas sings, gun in hand, before opening fire inside the Factory.It’s a bold choice for a movie director making theater for the first time, and Van Sant, 69, didn’t just contribute the script. He is also listed as the stage designer and composer of “Andy.” (Paulo Furtado, a Portuguese musician who goes by The Legendary Tigerman, is credited with the “musical direction,” as well as the arrangement for most numbers.)While “Andy” is an unexpected outcome, Van Sant has had a Warhol project in mind for over three decades. In the late 1980s, he developed a screenplay for Universal Pictures with Paul Bartel, in the hope that it would star the actor River Phoenix. After Phoenix died in 1993, the project was shelved.The invitation to turn to theater came from John Romão, the artistic director of Lisbon’s Biennial of Contemporary Arts (BoCA), which runs through mid-October. While “Andy” is performed entirely in English, the cast and crew are all Portuguese. After the initial run concludes in Portugal, “Andy” will tour around Europe, stopping first in Rome and in Amsterdam.Some tweaks may yet improve “Andy,” but let’s start with the obvious: Creating musicals is a craft. It would be miraculous to produce a good one on first try. Even though the Virgin Mary pops up onstage to banter with Warhol, “Andy” is no miracle.Diogo Fernandes, as Andy Warhol.Bruno Simão/BoCA Bienal de Artes ContemporâneasWhile Van Sant has spent much of his film career circumventing the Hollywood rule book, his approach here is relatively prudent. “Andy” has a clear narrative arc, spanning the years between 1959 and 1967, and the expected musical numbers for both soloists and small ensembles. There is even an attempt at choreography in an early scene, although the group’s hip thrusts when Warhol’s homosexuality is mentioned are less than subtle.If anything, however, the relative conventionality of “Andy” exposes Van Sant’s inexperience with the syntax of live performance. Entrances and exits give him away early on. Devising believable transitions is a basic conundrum of theater, and “Andy” is choppy, with actors coming and going uneasily.Warhol is also a paradoxical subject for a musical. Songs have a way of baring a character’s soul, but Warhol’s deliberately enigmatic persona has been difficult to parse, even for scholars. His transformation onstage from the bespectacled, painfully shy Andrew Warhola, who wears a bow tie and stalks Truman Capote, into the high priest of Pop Art produces something like whiplash. Suddenly, he becomes a hollow shell, who treats his Factory collaborators — including Edie Sedgwick — with utter callousness.Van Sant’s songs shy away from exploring his inner life from that point, focusing instead on artistic debates and one-off events like Solanas’s shooting. Musically, they are fairly even and flat, lacking in tunes that might carry the action; perhaps an injection of the Velvet Underground, the band Warhol once managed, might have helped.Surprisingly, the book also gives Warhol fairly little agency in his own career. His mother is credited with the idea for his soup can series. Gerard Malanga, Warhol’s only lover to appear in the show, gives him the makeover that lets him fit in with the New York underground scene. Later, he is portrayed as hapless with the business of running the Factory.Martim Martins as Gerard Malanga, left, with Fernandes as Warhol.Bruno Simão/BoCA Bienal de Artes ContemporâneasSome scenes and lines are lifted directly from TV interviews, including an appearance by Warhol and Sedgwick on “The Merv Griffin Show.” In others, characters fall victim to Van Sant’s clunky expository dialogue. Greenberg, an authority on modernism, may have despised Pop Art, but he surely deserved better than to sing: “I’m an extraordinary man, I expect extraordinary stuff.” Van Sant opted to work with a young, mostly inexperienced cast, and acting and singing in English is clearly a tall order for many of them, although they try bravely.The strongest overall performance comes from Helena Caldeira, who captures the restless allure of Sedgwick. As Warhol, Diogo Fernandes has less vocal range, but he pulls off Warhol’s two sides. One of the strongest scenes sees him earnestly asking the Virgin Mary: “Do you think Pop Art can be unholy?” As Mary, Caroline Amaral nails silly, wonderful quips, and their exchange suggests leaning into the bizarre might have turned “Andy” into a more Warholian proposition.Another brief flash of absurdity comes at the end, as Warhol is reunited with Capote in heaven. (Capote immediately asks where the gay bars are.) There is a flamboyant, preposterous comedy lurking within “Andy.” As of now, Van Sant lacks the theatrical tools to unleash it. More