Elision and interpolation
Directors must make artistic decisions about what to include and exclude from the source material. The original mediums have a significant influence on these decisions, for example, much must be elided in the adaptation from a novel to a stage production, due to practical time constraints. These decisions are always controversial and comparisons between the original and the adaptation are unavoidable.
The Phantom of the Opera was originally a novel by Gaston Leroux written as a serialisation from 1909-1910. It is the longest running show in Broadway history. There are numerous examples of novel adaptations in the field, including Cats (musical), which was based on Old Possum's Book of Practical Cats by T.S. Eliot and Les Misérables, which was originally a historical novel by Victor Hugo.
The Lion King, was originally a Disney animated film and its theatrical adaptation has become the most successful musical in history.