Robert Louis Stevenson told a Washington correspondent that his story of "Dr. Jekyl[l] and Mr. Hyde" had for its foundation an incident related to him by a London doctor who made diseases of the brain a specialty. None of his work was absolute fiction, and most of it had a basis in actual experience. "I do not believe," he said, "That any man ever evolved a really good story from his inner consciousness, unaided by some personal experience or incident of life."
If anyone knows more details, or a fuller version or an earlier appearance of this anecdote, I'd be grateful to learn of it.
Update 1/2/16: One reader wrote in with some information on "the doctor" in Jekyll and Hyde, not the "London doctor" of the quotation, which I copy below (thanks, Brian!). I remain primarily interested in the source of the quote (and especially whether there is any more context given, and any further Stevenson comments):
The doctor that Stevenson apparently based aspects of Jeykll/Hyde on is John Hunter, the famous Scottish surgeon, who's personal anatomical collection became the foundation of the famous Hunterian Museum (of the Royal College of Surgeons) in London; the Hunterian Society derives its name from him too. He had an older brother, William Hunter; also a famous Scottish anatomist; he also had a private collection (with books & coins & much else), which went on to form the nucleus of the renowned Hunterian Museum (& Art Gallery) of Glasgow University. Two incredible brothers, from an era of Glasgow/Scottish history that I'm reasonably familiar with.
(I haven't actually read the Wiki articles in detail; the links are just so you know who I'm referring to.)
Ian Rankin (the Scottish crime writer) made a documentary (which I didn't actually like very much) which covers the story of Stevenson & Hunter; in particular some aspects of Hunter's London residence, which will be very familiar to anyone who's read Stevenson's Jekyll & Hyde. Some of the details covered in the documentary are dealt with in the following article (from the Guardian, 2010): http://www.theguardian.com/books/2010/aug/16/ian-rankin-dr-jekyll-mr-hyde