How is sound from Front speakers changed when installing rears?

The sound is separated when it is mixed, see:

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.