The Mortal Instruments: City of Bones, the film adaptation of the first book in Cassandra Clare’s New York Times bestselling fantasy-adventure series, comes out next summer, but will the movie be as successful as the novel? The answer depends, in large part, on its cast.

The Mortal Instruments has already landed its two young stars – Jamie Campbell Bower as Jace Wayland and Lily Collins as Clary Fray – to the general approval from fans – and now the film has added three more to its cast.

In a recent post at her blog, Cassandra Clare announced the additions: Lena Headey (Dredd, Game of Thrones) as Jocelyn Fray, Clary’s mother and a Shadowhunter-in-hiding; Jared Harris (Sherlock Holmes: A Game of Shadows) as Hodge Starkweather, Jace and Clary’s mentor; and Godfrey Gao as Magnus Bane, the High Warlock of Brooklyn.

For the uninitiated, here’s a description of The Mortal Instruments: City of Bones:

In The Mortal Instruments: City of Bones, set in contemporary New York City, a seemingly ordinary teenager, Clary Fray, discovers she is the descendant of a line of Shadowhunters, a secret cadre of young half-angel warriors locked in an ancient battle to protect our world from demons. After the disappearance of her mother, Clary must join forces with a group of Shadowhunters, who introduce her to a dangerous alternate New York called Downworld, filled with demons, warlocks, vampires, werewolves and other deadly creatures.

Lena Headey in 'Game of Thrones'

Since I haven’t read the series, I can’t speak to whether or not any of these actors are a good fit for their respective roles. That said, Headey is one of the most in-demand actresses in Hollywood right now, and Harris just earned his first Emmy nomination for his work on Mad Men, so it’s safe to say that they’re solid additions to the cast.

And Gao? Well, he’s a good-looking model, so that’s probably worth something. Wait a minute, this is a young-adult fantasy franchise – that’s practically all that matters! (Just kidding TMI fans.)

Anyway, with The Twilight Saga: Breaking Dawn – Part 2 coming out this fall and finishing out the series, The Mortal Instruments could have the chance to be Hollywood’s next big young-adult movie franchise. It still has to compete with The Hunger Games, but the movies won’t face off directly.

The Mortal Instruments comes out on August 23rd, 2013 – while The Hunger Games: Catching Fire comes out three months later on November 22, 2013.