Johnny Depp in The Libertine

There may never have been a stranger movie star than Johnny Depp. For a decade and a half, he’s been cast as a rather wide variety of key eccentrics, oddball naifs and sympathetic grotesques. He rose to stardom in Tim Burton’s DayGlo fantasy, Edward Scissorhands, and went on through a number of offbeat, idiosyncratic movies, while remaining a star. It’s not clear how he’s got away with it, although being uncommonly handsome can’t hurt. He’s never made it to the first rank of stellar leading men, but this probably doesn’t concern him much, considering his choices. (He turned down Speed.) His manifestly obvious gifts and adeptness also seem to have earned him some latitude.

The offset from this unusual career trajectory is that Depp has never given a really important, seriously conceived, unassailably impressive performance, the kind Heath Ledger got down in Brokeback Mountain. And Laurence Dunmore’s The Libertine will do next to nothing to alter this record. It’s likely to be regarded, by the relative few who pay any attention to it, as a dismissible digression on the way to Depp’s appearance, later this year, in the sequel to his biggest box-office success, Pirates of the Caribbean.

The Libertine’s appeal to Depp’s fans is likely to be very limited. It’s a biopic about the 17th-century poet, dramatist and rakehell, John Wilmot, the Second Earl of Rochester, who gained fame and infamy during the first 20 years of the reign of England’s King Charles II, “The Merry Monarch.”

More than a century later, another poet, Lord Byron, was called “Mad, bad and dangerous to know,” but the description fits Wilmot even better. An audacious sexual adventurer (he kidnapped his eventual wife), he was a profligate and prodigal son, possibly until a reported—and disputed—repentance near the end of his shortened life (he died of drink and syphilis at 33). The great Dr. Johnson said, “He lived worthless and useless and blazed out his youth and his health in voluptuousness.”

There was a great deal more to him than that. He was a poet of lasting and high merit, a dramatic and poetic satirist and a public figure in Restoration England. He also was a darkly complex, probably tortured moralist, despite his scandalous reputation and conduct.

All this may well have appealed to Depp, who is reported to have indulged in some hotel-room rampages in earlier years. But this adaptation of Stephen Jeffreys’ play isn’t provocative, entertaining or relevant enough to involve audiences.

Writers’ lives aren’t promising movie material, anyway, except as they’re exciting when they’re not working. Wilmot surely lived large and licentiously, but making his era exciting and understandable for contemporary audiences would be difficult, even with a more accessible film. And the filmmakers haven’t made things easy for us. They’ve turned out a perversely off-putting, doom-ridden movie with a hard-to-follow narrative.

Their movie centers on Wilmot’s relationship with the actress Elizabeth Barry (Samantha Morton) and his efforts to coach her to stardom. This tack is based on a popular story about which historians have expressed doubts.

It isn’t really the film’s uneasy grasp of fact that’s the problem. The movie is humorless and eventually grim. Dunmore has gone out of his way to dampen its aesthetic appeal. He and cinematographer Alexander Melman have even darkened and filtered their images so that scenes often have a dull brown or sickly green-hued cast. This is supposed to reflect Wilmot’s graphically depicted decay, but this approach leaves no contrasting highlights, literally.

The young earl’s relationship with the king (John Malkovich, whose idea it was to film the play) might have made a more intriguing storyline than the heavy-fated one with Barry. Charles tolerated, promoted and was a patron of Wilmot, who remained stubbornly, sometimes insolently, disrespectful.

The film has Wilmot outraging the court and the French ambassador by staging for the King his scabrous satirical play in which a fictitious monarch decrees that sodomy is the only acceptable kind of sexual congress. (The play is real, the incident is not.)

The stars do surprisingly well with what little they were given. Malkovich is unexpectedly regal and quietly ironic. But though it appears Depp is working earnestly at creating a convincing portrayal, he doesn’t really succeed in suggesting the deeply disappointed, passionately dissatisfied man Wilmot must have been.

The Libertine is a dreary curiosity. And it provides another occasion to wonder if Depp’s gifts will ever engage a role that can elicit a serious, compelling performance.