David Cunningham is directing the adaptation of the Susan Cooper fantasy favorite, which will begin production at the end of the month in Romania.
The story focuses on a boy (relative newcomer Alexander Ludwig) who discovers when he's 11 that he's the last of the Old Ones, immortals entrusted with saving the world from the forces of darkness.
According to The Hollywood Reporter, Smith will play the boy's rebellious older brother, while Eccleston will take a dark turn as Rider, the film's villain.
Other new additions to the cast include Amelia Warner ("Aeon Flux"), Emma Lockhart ("Batman Begins"), Gary Entin, Edmund Entin, Jonathan Jackson and John Benjamin Hickey ("Flags of Our Fathers"). Those actors join the previously announced Ian McShane and Frances Conroy.
Smith is best known for his starring turn on The WB's beloved "Everwood." His feature credits include "Nearing Grace" and the upcoming "Closing the Ring."
Eccleston has been visible as an invisible man on NBC's "Heroes," which boosted the actor's already strong fanboy cred from roles in the BBC's "Doctor Who" and the zombie classic "28 Days Later."