Johnny Depp helped a girl "mutiny" against her teachers, in response to a letter.

Her letter in full:

Captain Jack Sparrow,

At Meridian on old Woolwich Road we a [sic] all a bunch of budding young Pirates. Normally we’re a right handful, but we’re having trouble mutinying against the teachers! We’d love it if you could come and help!

From

Beatrice Delap, aged 9, a budding Pirate

PS we have a plentiful supply of Rum!

Depp Captain Sparrow was inclined to acquiesce Ms. Delap's request, so he showed up to her school for the mutiny, dressed in full pirate garb, no less. At the time, Depp was shooting scenes at nearby Old Naval College for the new Pirates of the Caribbean movie. The school was only notified about the appearance 10 minutes before the movie star arrived, but they were good sports about the whole thing. Sound like a cool school; I wish my school had a “plentiful supply of rum”.
(source)