Cisco Firewall :: 5500 ASA Modular Policy Framework

I understand from the Cisco documentation that a service-policy applied to an interface on an ASA 5500 series firewall, will override the default global service-policy.  However, I am not clear on whether it will override the entire global service-policy, or only the parts where they overlap.  In other words, would the resulting service-policy on the interface in question be just what was applied in the service-policy on the interface, completely replacing the global service-policy?  Or, would it be a combination of the global and interface service-policies, with the interface one taking precedence where they overlap?
 
if I wanted an interface to have the same service-policy as the global service-policy plus on other item, can I just add the one item in a service-policy that I apply to the interface, or do I have to replicate all the items from the global policy, plus the one additional item, and apply that to the interface.