Hasnat Khan (Naveen Andrews) and Princess Diana (Naomi Watts) steal a moment alone in “Diana.” (Laurie Sparham/EOne) Hasnat Khan (Naveen Andrews) and Princess Diana (Naomi Watts) steal a moment alone in “Diana.” (Laurie Sparham/EOne)

When it came to his leading lady and the royal subject of his biopic “Diana,” which opens Friday, director Oliver Hirschbiegel had one small problem.

“In real life, Naomi [Watts] doesn’t really look like Diana at all,” he says. “But very excellent actors, they have that in them. I don’t know how they do it. They change their features; they become the character from within.”

In Watts’ case, she had to become a different Diana, Princess of Wales, from the one we all think we know. The film, based on Kate Snell’s book “Diana: Her Last Love,” traces the final two years of Diana’s life, during which she dated heart surgeon Hasnat Khan (Naveen Andrews).

Never heard of him? That’s because Khan and Diana managed to keep their relationship largely under wraps, the book and movie allege, through a combination of outright denials and clandestine actions: Diana wore wigs when they went out and smuggled Khan into Kensington Palace in the trunk of her car. (Diana had been separated for nearly three years but was not yet divorced from Prince Charles.)

The relationship, Hirschbiegel says, was largely what prompted Diana to take a larger role in her charity work, most notably in the push to clear and ban land mines. Through Khan, the German director says, Diana came to understand that most people don’t spend their days princessing about.

“She started standing for something, fighting for something, and having a real purpose in life — aside from being the most famous woman in the world,” Hirschbiegel says.

In the film’s smaller moments, Watts portrays Diana as just an ordinary woman out to impress a man. Before Khan comes over for dinner, for instance, Diana gets a friend to do the cooking. When Diana reheats the meal for Khan, she hovers by the microwave to stop it before the beep of the timer can give her away.

Eventually, Khan seems to forget that Diana is who she is. “Whenever they are alone, [her position] is out of the way,” Hirschbiegel says. “They just enjoyed each other tremendously. They had a lot of fun with each other. On top of that, they both are what you would call old souls, so that’s another level where they connected. When they are together, all the other things don’t matter anymore.”