Complex sentence: Producers of the show initially approached Michaela McManus to play Hannah, but later, Allen was cast instead, while McManus obtained the role of Tara, Rex's tennis coach, which Allen had originally auditioned to play.
Split into simpler sentences: Producers of the show initially approached Michaela McManus to play Hannah. However, later, Allen was cast instead, while McManus obtained the role of Tara, which Allen had originally auditioned to play.