In How to Train Your Dragon 2 (due June 13), the Oscar-winning actress voices Valka, a maverick Dragon Rider who lives as a recluse in the Arctic and becomes a mentor of sorts to the young Viking hero of the first movie, Hiccup (Jay Baruchel).
Blanchett's sons were fans of the first film, which earned more than $500 million worldwide. So she was intrigued when DeBlois approached her out of the blue at the Oscars ceremony, where Dragon was nominated for best animated feature and best original score.
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"She was just standing there," the director recalls, "and I said, 'I've written a part for you in How to Train Your Dragon 2.' You have to seize those opportunities when they present themselves. She's such a glamorous starlet but, at the same time, this very grounded, super-cool Australian who drinks out of the can."
DeBlois' follow-up checks in with Hiccup and his dragon pal, Toothless, five years after he brought humans and dragons together on the island of Berk.
An outsider who's now the village hero, Hiccup is on the verge of adulthood, with his chieftain father, Stoick the Vast (Gerard Butler), grooming him for responsibility. But he'd rather embrace what's left of his carefree childhood.
Hiccup's sense of adventure drives him to explore the world past Berk, and he and Toothless travel to the Arctic and stumble upon Valka, a vigilante who is springing dragons from traps and hiding them in a refuge away from the villainous dragon hunter Drago Bludvist (Djimon Hounsou) and his No. 2, Eret (Kit Harington).
Reprising their roles from the first film are Kristen Wiig, Jonah Hill, Craig Ferguson, America Ferrera and Christopher Mintz-Plasse.
Valka was designed to be for dragons what Dian Fossey was for apes, DeBlois says: "Somebody living with the animals and learning to communicate with them in non-verbal ways and becoming much like a dragon herself.
"Her years of isolation and dealing with nothing but dragons and being very sympathetic to them has made her very distrusting of humans."
Hiccup considers Valka to be a guru in a way, and he is wowed by the relationship between her and her dragon Cloudjumper. While Toothless was created to look like a black panther, Cloudjumper has owl mannerisms and a second set of wings under his main ones that "can actually splay and become an X-wing," DeBlois says.
Hiccup is largely a pacifist, however, and can't grasp all of Valka's ideas, including hiding dragons and keeping them from coexisting with mankind.
"He both admires her but sees her as what his life could be like taken to the extreme," DeBlois says.
"Valka's life is not only learning about all (the dragons') secrets and ways and living among them, but also playing among them," he adds. "That's part of what she can show Hiccup, things he's never known about dragon behavior.''
Blanchett has been DeBlois' favorite actress since she played the queen in 1998's Elizabeth, but for Dragon, he says, "we were able to tap that fiery strength that is present in every word that she utters and put it on the screen."