Lasse Hallstrom makes trifling, negligible movies that are often good but seldom of any consequence. (This doesn’t stop them from being treated by some people as if they were consequential, but that’s another matter.) “The Cider House Rules,” “Chocolat,” “The Shipping News” and 2005’s “An Unfinished Life” are all competent, enjoyable films with a smattering of emotion and a dash of soft-focus romance.

But now comes “Casanova,” in which Hallstrom leaves mere lightness behind and goes into full-fledged farce. Written by Jeffrey Hatcher and Kimberly Simi, this isn’t so much a biography of the legendary Italian lover as a fictionalized episode of something that could have happened (but didn’t) one winter in Venice.

Casanova (Heath Ledger) is a nobleman known throughout Europe as an expert seducer of ladies. This does not sit well with the Catholic Church, which in 1753 is still allowed to execute people for committing sins like fornication. The doge of Venice is a friend to Casanova, and warns him he’ll either have to leave the city or clean up his act. Casanova doesn’t want to leave Venice (he’s still waiting for the mother who ditched him when he was young to show up and reclaim him) so he proposes marriage to a virgin named Victoria Donato (Natalie Dormer).

Problem is, Victoria is loved deeply by Giovanni Bruni (Charlie Cox), the lad who lives across the canal from the Donatos. And Casanova, as soon as he spies Giovanni’s feminist sister Francesca (Sienna Miller), knows he settled on virgin Victoria too quickly. But Francesca couldn’t be wooed by Casanova even if she didn’t think he was a cad — she has a fiance (whom she has never met) in Genoa due to arrive any day.

And thus the stage is set for disguises, mistaken identities, women dressing up as men, double-entendres, and those conversations where the people are talking about different things but think they’re talking about the same thing. It’s a merry circus with the cross-dressing and high stakes of a Shakespeare play (Casanova almost gets hanged!) and the lowbrow plots of a French farce.

You can see why Ledger chose this role after “Brokeback Mountain”: Nobody is more famously heterosexual than Casanova. And Ledger gives the libertine an agreeably rakish air, less a serial lady-killer than a naughty little boy (albeit one in his 20s).

Yet something is amiss. Despite the scene-stealing charms of Oliver Platt as the Genoese fiance (he’s a businessman who’s made his fortune in lard), and the scenery-chewing over-the-topness of Jeremy Irons as the villain Bishop Pucci, not to mention Lena Olin (Hallstrom’s wife) as Giovanni and Francesca’s mother, the whole thing feels too mindlessly cheerful. It has an “all’s well that ends well” sort of finale, with every eligible character finding love and fulfillment while the bad guys are thwarted. It’s pleasant, but it’s predictable, far too airy to be remembered.

C+ (1 hr., 48 min.; R, a lot of double-entendre and naughtiness, brief fairly strong sexuality.)