He acknowledged the problems inherent in portraying such a beloved figure and said that during shooting he'd run foul of the leading Sherlock Holmes expert, who was advising on accuracy.
Seeing that the guy was very upset about something, Downey asked what he had done wrong.
Turns out that whilst in character as Holmes, Downey called Watson (Jude Law) "John," which is a huge no-no. The disappointed expert told Downey that Holmes would never call his trusty sidekick by his first name: "Only call him 'Watson' or 'Mr. Watson.'"
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