Fans of The Mortal Instruments book series can rest easy: Lily Collins, who’ll bring Clary Fray to life on screen, is just as obsessed with Cassandra Clare’s books as you are. (“Join the club!” she says with a laugh.) Collins, who was cast as Fray over two years ago, has been a fan of Clare’s work for quite some time. Now the story, which centers around a young girl who discovers a brand-new (to her) supernatural world, is finally coming to the big screen this summer, with a cast that includes Jared Harris and Jonathan Rhys Meyers.

“The special effects and the CGI are going to be...