At the time “Spencer” is set, Princess Diana was already a mother, and her two sons, Prince William and Prince Harry, play an important part in the film. Despite spending extra time with the actors playing the two princes in order to build the maternal bond Diana shared with her two sons, Stewart says this lack of experience in motherhood was the only way she felt disloyal to Princess Di in her performance. The boys were only 15 and 12 when their mother died.
Nevertheless, Stewart did her best to capture how “untouchable” Diana seemed when she was with her children, telling the Daily Mail, “She’s like this feral animal you wouldn’t dare accost.” Despite her reservations about playing a famously loyal mother, Stewart said she hoped to capture the contradictions at the heart of the character. “It would’ve been easy to make her kind of perfect and it’s so fun to martyr her because she’s beautiful and just wanted people to be happy,” Stewart said, but the truth of the matter is that Diana embodied “the weirdest combination of things that don’t necessarily go together all the time.”