According to Yu, the young Jackie played a variety of roles, including some female parts, but he specialized in the painted face roles of the opera clowns.
which led me to this Wiki article about the Beijing Opera and this quote:
The Wu Chou is one of the most demanding in Beijing opera, because of its combination of comic acting, acrobatics, and a strong voice. Chou characters are generally amusing and likable, if a bit foolish.
as well as this one:
Beneath the whimsical persona of the Chou, a serious connection to the form of Beijing opera exists. The Chou is the character most connected to the guban, the drums and clapper commonly used for musically accompaniment during performances. The Chou actor often uses the guban in solo performance, especially when performing Shu Ban, light-hearted verses spoken for comedic effect. The clown is also connected to the small gong and cymbals, percussion instruments that symbolize the lower classes and the raucous atmosphere inspired by the role. Although Chou characters do not sing frequently, their arias feature large amounts of improvisation.
Recognise Jackie's onscreen persona anyone?
The emphasis on improvisation as a characteristic of the clown is really interesting because it is the ability to improvise that has to be Jackie's most recognisable trademark - along with his incredible physical ability of course.
The connection with drums is also interesting as Jackie has also carried this into his adult life. We can see clips of Jackie playing the drums here and here. He also played drums to open his concert at the Birds Nest Stadium earlier this year.