Calling base methods

The CallsBaseMethod configuration method can be used to make a method execute the implementation of the faked class:

A.CallTo(() => fakeShop.SellSmarties())
 .CallsBaseMethod();

Configuring a method to call its base method only makes sense if the method is actually implemented, so this technique cannot be used on an abstract class method or on any method from a faked interface—a faked abstract method told to call its base method will throw a NotImplementedException.

Configuring all methods at once

Perhaps you want to have all or nearly all of a fake's (fakeable) methods defer to the original implementation. Rather than using CallsBaseMethod a dozen times, the fake creation option CallsBaseMethods can do all the work at once:

var fakeShop = A.Fake<CandyShop>(options => options.CallsBaseMethods());

And then selectively override some of them

A.CallTo(() => fakeShop.SellRockets()).Throws<Exception>();

Default interface members

A fake may also be instructed to execute the default implementation for an interface member using CallsBaseMethod or CallsBaseMethods as above.

This will not work for explicitly-implemented interfaces that have default member implementations

Explicit interface implementation redeclares the implemented interface's member(s), making them effectively new methods on the type, so there is no "base member" implementation to call.