Why did renowned English actor Alan Rickman choose to remain in the "Harry Potter" film franchise even after being diagnosed with prostate cancer in 2005? At the time, "Harry Potter and the Goblet of Fire" had just been released, the fourth instalment in the main film series.

The upcoming release of a collection of his handwritten diary entries, "Madly, Deeply: The Diaries of Alan Rickman," reveals answers to this question and more. An excerpt from his upcoming posthumous book divulged that Rickman felt obligated to see his role as "Professor Severus Snape" after J.K. Rowling revealed a vital aspect of Snape's character to him in confidence.

In a diary entry dated 2007, Rickman indicated that the revelation that Snape deeply loved Lily Potter, as told to him early on by Rowling, "gave me a cliff edge to hang onto." He wrote, "Finally, yes to HP 5. The sensation is neither up nor down. The argument that wins is the one that says: 'See it through. It's your story.'" prior to the start of production for "Harry Potter and the Order of the Phoenix."

In the same year, Rickman also wrote, "I have finished reading the last 'Harry Potter' book. Snape dies heroically, Potter describes him to his children as one of the bravest men he ever knew and calls his son Albus Severus. This was a genuine rite of passage. One small piece of information from Jo Rowling seven years ago – Snape loved Lily – gave me a cliff edge to hang on to."

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Variety shared that Rickman kept his handwritten diary entries with the intention of publishing them in the future. By the time of his death in 2016, he had amassed a total of 27 volumes. Henry Holt and Company collated and edited his words to create a single-volume collection that will be released on October 18.

Alan Rickman died on Jan. 14, 2016, of pancreatic cancer. He was 69 years old.

Severus Snape played by Alan Rickman
Warner Bros