Recently, the cast members for The Host have been announced. It seems that Saoirse Ronan will be playing Melanie Stryder. You probably know her best as Briony from Atonement, or Susie Salmon from The Lovely Bones. She also played Hanna in the film by the same name. Alongside her, Max Irons will play Jared Howe.
Her younger brother Jamie Stryder will be played by Chandler Canterbury, who is known for playing eight-year-old Benjamin Button in The Curious Case of Benjamin Button. For the rest of the cast for The Host, go to the film page here.