Shakespeare In Love is a variation on the mean old Hollywood gag about the actress who was so dumb she slept with the writer. As Romeo And Juliet goes into rehearsal in 1593, someone asks the producer who this Will chappie is. "Nobody," comes the reply, "he's the author."

The twist here is that the actress - played by Gwyneth Paltrow - is smart enough to know that the writer is a force to be reckoned with. But the fact that he's Shakespeare doesn't make him any less of a buffoon. Scrabbling for finance, relying on borrowed ideas, this chancer bustling round the Southwark taverns is, right down to his funky scuffed leather jerkin, kin to the wannabe screenwriters of LA or Soho today.

Acclaimed as the wittiest romantic farce since All's Well That Ends Well, and garlanded with Golden Globe awards, Shakespeare In Love has all the bases covered. It's a jovial demystification of English history, it's a backstage comedy full of conniving impresarios and inflated thesps. And to tickle Hollywood's fancy - this being a British film in spirit but a Miramax production in reality - it's also a movie-biz satire, thinly disguised in Elizabethan fustian. Then, as now, everything hinged on pulling in the punters. "You see," explains Geoffrey Rush's seedy theatre manager, "comedy, love and a bit with a dog - that's what they want." And then, as now, you were risking your neck by doing what Shakespeare did in Romeo And Juliet - opting for a downbeat ending.

Comedy, love and a bit with a dog is exactly what we get. Most of the humour in Shakespeare In Love relies on anachronism, sometimes of a strictly Flintstones variety: "I had that Christopher Marlowe in the back of my boat once,'' proclaims a Thames oarsman. However, the film is determined not just to make you laugh, but to get you sobbing too - if it worked in Twelfth Night, chances are the formula still holds up.

The Bard's muse proves to be a noble maid who dreams of acting but knows she'll only get on stage by dressing as a boy. Posing as hopeful young player "Thomas", Viola lands the part of Romeo, while assorted blackguards and killjoys (Colin Firth, Tom Wilkinson and Simon Callow) stand by to ensure their love is star-crossed.

Disappointingly, the sexual masquerading is scrupulously downplayed in favour of by-the-book romance. Paltrow dons a rather fetching wisp of moustache, and Will starts confessing to "Thomas" his love for Viola. But the gag's not followed through - she unmasks herself instantly, and it's buskins off for a tastefully torrid idyll.

Soon, Will seizes every opportunity to coach Thomas in kissing technique during rehearsals - but such campery is nipped in the bud before it becomes an Elizabethan Cage Aux Folles. So much for what might have been the film's most authentically Shakespearian source of humour.

Shakespeare In Love is determined to play it safe and please everyone. It's a remarkably astute packaging exercise, with in-jokes for the literati and even for fans of director John Madden's last film Mrs Brown - Judi Dench knowingly plays Elizabeth I as a variant on that film's Victoria. The cast has legit Shakespearians, a few friendly Britcom faces, and for US indie-cred, Ben Affleck, rather jaunty as a swaggering proto-luvvie.

The real cultural clout comes from Tom Stoppard - brought on board, say the press notes, "to add his own magical touch" to the work of American writer-producer Marc Norman. Presumably Stoppard is to thank for the jollier one-liners and for the interplay between life and art, as Will and Viola's romance starts generating good stage ideas. But though there's no shortage of well-turned gags, the film doesn't actually feel much like a comedy, and the fault isn't necessarily Madden's.

Whether or not he has a funny bone in his body is hard to tell - after all, he does get the actors to swagger and swoon with requisite gusto. The problem may lie more in the chosen form, despite all the efforts to puncture the bubble of historical accuracy, this is absolutely mainstream costume romance. Every last codpiece is given the painstaking period look; the muddy streets feel researched to the last wisp of straw. The earnestness that invariably attends such re-creations only stifles the humour. Why not take a lead, instead, from the hyper-stylised feel of Olivier's Henry V and really dare us to suspend disbelief?

What we get is an all-out attempt to dazzle us with English Heritage prestige. When Will's play is finally performed, the camera wheels, the music surges, and we're invited to gasp at the Birth Of A Legend.

Fiennes and Paltrow at least play the glamour ticket with considerable charm. Looking like a Nicholas Hilliard miniature, Fiennes gives Will just a dash of Errol Flynn, stopping short of sending himself up too rotten. And Paltrow, honing her Sliding Doors drawl, is wryly unselfconscious, convincingly slipping in and out of a chatty version of blank verse.

But the film doesn't wear its conspicuous cleverness lightly. This sort of theatrical conceit needs to be dashed off with the flair of a Lubitsch, or at least the flip dryness of Woody Allen's Bullets Over Broadway. Sniping at what's already been celebrated as a major national triumph always looks not-quite-cricket, but Madden's film is transparently one of those things that the British Do So Well, and the Americans fund so handsomely. It sets out to provide more than your money's worth, but Shakespeare In Love falls some way short of infinite jest.