Not long ago, Harald Zwart was photographed sporting a large neck tattoo of a rune. “That was just for the day,” the director said, laughing. He had worn the tattoo to the Los Angeles premiere of his new movie, The Mortal Instruments: City of Bones, and he had tweeted a photo of it with the provocative question, “Who can name the rune I’m wearing?” The answer, judging by the rather impassioned replies, was apparently “fortitude”.

The night before the L.A. premiere, Zwart visited Mortal Instruments fans camped out at the theatre. “They were all lying outside in their tents,” he said. “So my wife and I drove up there and handed out warm soup to them.” Now the director, who grew up in Norway, was calling from Toronto, where he said he had been scouting locations for the next film, City of Ashes. Cue the shrieking.

Emotions run hot for novelist Cassandra Clare’s The Mortal Instruments. (City of Bones the movie opens this Friday, August 23.) And why not? It is a young-adult fantasy series about New York teenagers called Shadowhunters who have angelic powers, hunt demons, and are hip and attractive in a Buffy the Vampire Slayer way. When Jamie Campbell Bower was cast as Shadowhunter Jace, dismay over the British actor’s perceived lack of Jace-ness rippled across Twitterdom. What director needs this sort of trouble?

Maybe there was something about Clary, City of Bones’ heroine (played by Lily Collins), who confronts demons and a missing mother with admirable fortitude. “I lost my own mother when I was very young,” Zwart said, “so I could just totally relate to the idea of a girl who, suddenly her mom disappears and how do you deal with that? You go into machine mode and you start thinking, ‘Okay, I put my own emotions aside and I’m just gonna focus on reality.’ But then what happens when the reality that you start clinging onto is not the reality you thought it was?”

When Zwart was a child, “we had a basement in our house that I was really afraid of.” In the movie, when two bad Shadowhunters are hunting good ones, one asks the other whether he wants to take the upstairs or down. “And the big guy, the biggest of them all, says, ‘Well, I’m not too crazy about the basement.’ I just came up with that right when we were shooting. I thought, ‘They gotta have a little moment.’”

There was also the Johann Sebastian Bach moment. Musing about exactly why Jace plays the piano, Zwart’s own father’s Bach enthusiasm, and demons responding to certain frequencies, the director suddenly recomposed the great composer. “ ‘Maybe Bach was a Shadowhunter! That could be a good joke, right?’ So I ran over to the painting right before we shot it and I added those little tattoos on his neck.”

Does he really put the flag of his hometown soccer team, Fredrikstad Fotballklubb, in all his movies? “I try to hide it more and more, for each movie,” he said, laughing. Any hints about the flag’s whereabouts in City of Bones? “No! No! You have to look for it and we do a huge competition in Norway. That’s gonna happen next week and we’re hoping all the Norwegians will discover it!” Right: so it’s somewhere between Bach and the basement then.