Duchess Kate gets teary during "War Horse" premiere

(CBS) Steven Spielberg says his new film "War Horse" brought the Duchess of Cambridge to tears at the London premiere on Sunday.

Pictures: William and Kate attend "War Horse" premiere
Kate attends gala premiere for "War Horse," will celebrate 30th birthday quietly

The director told the BBC that his wife, actress Kate Capshaw, had to pass Kate a tissue during a pivotal moment in the film at the premiere.

"Right in front of my face she [Capshaw] passed a Kleenex to the Duchess of Cambridge, but I didn't want to ruin her experience of watching 'War Horse' so I never glanced over," said Spielberg, who had the duchess to his left and his wife on his right.

The former Kate Middleton celebrates her 30th birthday on Monday. She and Prince William will commemorate the day with a "low-key and private" affair, according to palace officials.