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Daniel Handler's A Series of Unfortunate Events: In The Film of the Book, "Let me try that again, quickly, while it's fresh in my mind." The dialog was supposed to end after Klaus says "Our parents just died", but Carrey felt he didn't get the reaction right. Silberling just kept the cameras rolling and Carrey ad-libbed from there.
R. A. Salvatore was working on a Forgotten Realms novel starring a barbarian named Wulfgar. His editor called and told him the book needed a sidekick... now. Salvatore panicked for a moment, then says, "A drow... Yeah, a drow ranger... Drizzt Do'Urden, of D'aermon N'achezbaeron, Ninth House of Menzoberranzan." Thus began the life of the dark elf that launched a million copycats (and mispronunciations).note http://www.rasalvatore.com/sidemenu/sampleChapters/de.html
When Louis Sachar was writing Holes, he used Stanley Yelnats as a filler name, planning to replace it later with a more normal name. He never did, and the weird name even ended up becoming a plot point.
James Joyce was plagued by problems with his eyes. This led to a frequent need to employ the use of dictation when writing Finnegan's Wake. During one session, someone knocked on the door and Joyce said, "Come in," which the typist accidentally added to the text. Joyce told him to keep it.
Commander Vimes from the Discworld: thrown together from clichés in order for Carrot to have someone to interact with on his arrival at Ankh-Morpork, he ended up taking on a life of his own and becoming one of the most interesting and important characters in the series.
In the James Bond novels, Bond originally drove a Bentley, which was destroyed in Moonraker. As Ian Fleming was working on Goldfinger, he recieved a letter saying "I hope you have the decency to fix Bond up with a decent bit of machinery - I would suggest an Aston Martin". The Aston went on to become the iconic Bond car. He switched Bond's Weapon of Choice from a somewhat underpowered .25ACP Beretta to the iconic Walther PPK in similar circumstances, even partially basing the initial characterisation of Q on the author of the letter.
Fleming was also impressed enough by Sean Connery's performance in the movies that he made Bond canonically Scottish.