Heath Ledger, starring as the fabled 18th-century Venetian lover, meets his match in a spirited, protofeminist beauty (Sienna Miller). Lasse Hallstrom (Chocolat) directs a sparkling screenplay by Jeffrey Hatcher (Stage Beauty) and Kimberly Simi; it starts as a frothy boudoir comedy but evolves into a masquerade by turns sweetly meditative and sharply satirical. Oliver Platt is hilarious as “Il Lardo,” the pig-fat merchant engaged to Casanova’s ladylove, and Jeremy Irons oozes reptilian malice as a pompous Catholic inquisitor thwarted by a series of increasingly complicated impersonations. Lushly romantic and forgiving of human foibles, this sumptuous production offers pleasures both earthy and ethereal. With Lena Olin and Natalie Dormer. R, 108 min.