Complex sentence: First-time screenwriter Linda Woolverton was hired to write the film's screenplay; this was the first time a screenwriter had been assigned to write a Disney animated film which were, prior to this point, written in storyboard format.
Split into simpler sentences: First-time screenwriter Linda Woolverton was hired to write the film's screenplay. This was the first time a screenwriter had been assigned to a Disney animated film which were, prior to this point, written in storyboard format.