Sienna Miller has been cast as Heath Ledger’s lady love in Lasse Hallstrom’s “Casanova.”Miller, who will make her bigscreen debut in Par’s forthcoming “Alfie” remake, joins Ledger in the Touchstone romance about the famed seducer who learns the meaning of true love when he meets a woman immune to his charms. An August start date is slated for the pic, which will lense in Venice. Mark Gordon, Leslie Holleran and Betsy Beers are producing. Kimberly Simi wrote the original script; Michael Cristofer redrafted it. Miller, known Stateside for her gig on TV’s “Keen Eddie,” stars in the feature “Layer Cake,” which will mark the directing debut of producer Matthew Vaughn (“Snatch”).