In which movie did Heath Ledger play the character Sir William Thatcher?

Heath Ledger played Sir William Thatcher in the 2001 film A Knight's Tale.

The character begins as a poor English squire who disguises himself as a knight so he can compete in jousting tournaments. With help from his friends Roland and Wat, and from Geoffrey Chaucer, he adopts the fictional identity Sir Ulrich von Liechtenstein of Gelderland. Ledger's William is determined to rise beyond the social class into which he was born.

A Knight's Tale was written and directed by Brian Helgeland and deliberately mixes a medieval setting with modern rock music, dance styles, and comic anachronisms. Paul Bettany plays Chaucer, while Rufus Sewell portrays William's rival, Count Adhemar of Anjou.

The name can cause confusion because William competes under the alias Sir Ulrich for much of the film. Near the conclusion, Prince Edward recognizes his ability and honor and formally knights him as Sir William. Thus, A Knight's Tale is the canonical answer, even though the character is often simply called William Thatcher.