Like nearly all the best ideas, the long running Basil Rathbone and Nigel Bruce Sherlock Holmes series, owes its origins to the powers of alcohol.
As Hollywood legend has it, producers Darryl F. Zanuck and Gene Markey attended a cocktail party where they were accosted by a drunken lady insisting they brought Holmes to the big screen. Zanuck joking teased "But who would play Holmes?", To which Markey interjected "why Basil Rathbone of course!". Rathbone's reported presence at the party possibly brining him to the fore in Markey's mind.
With the benefits of time, Rathbone was an obvious choice to play Holmes; his aquiline features, his frame, his voice, his presence, but at that point this was an actor heavily typecast…