Hey. In modern surround (post Dolby ProLogic) the 5.1 channels are discrete. That is, there's no real crosstalk or rebalancing happening that wasn't put there by the recording engineers.
Typically though, center is used for the majority of dialogue. L, R and Surrounds are left for music, and surrounds also do effects and ambiance (crowds, forrest, rain, etc). Planes flying overhead, bullets whizzing by, etc all rely heavily on the surrounds.