Andrew Garfield and Heath Ledger had a friend in common, so Garfield wasn't surprised when Ledger showed up at his 24th birthday party. What did surprise him was Ledger's gift: news that Garfield would be Ledger's costar in Terry Gilliam's surreal The Imaginarium of Doctor Parnassus (opening Dec. 25). He'd never even told Ledger he'd auditioned. ''He came to my party, gave me this really big hug, and said, 'Welcome to the family,''' says Garfield, now 26. But the two actors competed on set, playing rivals for the attentions of the fatherly mystic Parnassus (Christopher Plummer) and his comely daughter (Lily Cole). ''We wanted to challenge each other, and Heath was so strong,'' says Garfield (Lions for Lambs). ''He made me fight harder than I ever had, and I'm so thankful to him for it.'' Ledger's death midshoot left a huge void, filled on screen by Johnny Depp, Jude Law, and Colin Farrell. On the set, ''it was like limping,'' recalls Garfield. ''A limb had been severed from all of us.'' The finished film, which Garfield first saw at Cannes in May, was a welcome salve. ''It made my jaw hit the floor,'' says the actor, who just started work on David Fincher's Facebook movie, The Social Network. ''I grew up watching Terry's films, and it was a special moment realizing I'm in one of those films.''