The trailer for "Excalibur" looked sensational: action-packed, pictorially dazzling, a little racy. Although director John Boorman had developed an astonishing affinity for fiascoes ("Zardox," "Exorcist II: The Heretic"), he possessed filmmaking talent, a sense of imagery that could be lyrically exciting. The Arthurian legends also seemed like a relatively safe, sane project for a director with a potentially ruinous visionary streak.

But in "Excalibur," opening today at area theaters, Boorman can't seem to master the ironic approach to high adventure that allows a movie to satisfy heroic longings without getting ridciculous. This stilted reenactment of the Arthurian saga finds Boorman evolving into a modernist parody of Cecil B. De Mille, whipping up a kitschy custume spectacle.

The principal comic provocation is the dialogue. Although pictorial spectacle is Boorman's strong suit, "Excalibur" turns out to be an absurdly talky chronicle. The scenario begins with Arthur's illegitmate conception in a tryst arranged through the magical connivance of Merlin (Nicol Williamson in an ostentatiously bad performance, suggesting Monty Woolley as he might have been parodied by Charles Laughton in a whimsically hammy mood), and plods ceremoniously through the familiar highlights of his emergence as a sovereign and the rise and fall of the Knights of the Round Table. Boorman would have been better off working in silence. It's difficult enough for the actors to maintain their dignity when dressed in shiny, spiky suits of armor and slinky harem gowns. They stand no chance against the inflated idion inflicted on them by Boorman and co-writer Rospo Pallenberg.

De Mille himself might envy the Round Table exchange between Helen Mirren, an intermittent kick as the evil, scheming sorceress Morgana, and Williamson as the crafty but ultimately unwary Merlin:

"Your eyes never leave me, Merlin. Perhaps you ask for what you've never known."

"Perhaps you lust for what you cannot have."

The poor actors keep dropping howlers that could only function effectively within the framework of a medieval spoof.

The ongoing verbal inanity is supplemented by amateurish bits of acting, preposterous conceits and wacky illustrative details. For example, Boorman has cast his own daughter, Katherine, as Arthur's sorely deceived mama, Igrayne, the Duchess of Cornwall. The first unintentional side-splitter in the show is her impromptu hootchy-kootchy dance, a clumsy twirl that supposedly inflames the assembled knights, who bang their sword-hilts on the banquet table in raucous horny unison. "I must have her!" pants Uther Pendragon, Arthur's lusty progenitor, and so he does.

Arthur and his knights look bizarrely doltish. A handful of Vikings would surely make mincemeat of this unimpressive collection. The only luster in Boorman's Camelot comes from the hghly polished surfaces of the gaudy armor and the gargantuan Round Table. Nicholas Clay's Lancelot is especially silly: when Boorman shows a group of maidens giggling at him as he advances on horseback, beaming that dopey smile, you'd swear tht the filmmaker had caught on, too.

No such luck. Boorman's style is too portentous to be deliberately funny.

Indeed, the only obvious comic touches are fleeting bits designed to portray Merlin as a lovably dotty old wizard. Boorman and Pallenberg probably picked up this motif from "The Once and Future King," but they have no aptitude for it.

The belated comic relief can't possibly complete with the amusement already provoked by contemplating Merlin's silver skullcap, whichmakes Williamson resemble Charles Middleton as Emperor Ming in the Flash Gordon serials, or by listening to his vocal gyrations, which flutter between a froggish croak and a birdish twitter, probably out of a desire to suggest Merlin's Oneness With Nature.

The working title of the film was, in fact, "Merlin." Boorman has acknowledged his fascination with the character in words that describe his own artistic dilemma: "He's a mixture of real awesome power and foolishness. He gets things wrong . . . He has enormous power and knowledge, and yet there are simple things he doesn't grasp or understand."

Exactly. In Boorman's case the contradictions emerge in the opening sequence and keep merrily gumming up the mythic-heroic works. There's a magnificent composition of mounted knights appearing on the crest of a hillside backlit in a beautiful, smoky golden hue. But when the battle begins, the sequence somehow loses its majesty.

Setting an unfortunate pattern for all subsequent battle scenes, it's an inconclusive round of banging and clanging. Moreover, one of the clangs -- the sound of a battle axe connecting with the top of a helmet -- sounds oddly funny. The tone reverberates in a way that rings all wrong for the purposes of martial romanticism. (In a similar respect, wounds have a curious way of spurting a blood substitute that looks less than authentic -- closer to cherry soda, in fact.) That's the problem of "Excalibut" in a nutshell: Boorman keeps swinging away in a heroic frenzy, only to produce those deflating, nutty little clangs.