Sorry, I misunderstood what you wanted.
In general a small example demonstrating what you need will go a long way in helping us understand you better.
Anyway, here is an example that shows how to mock a specific method from on a given instance. (In this example the method is declared on the parent class, but this will work on on all other methods as well)
public class BaseClass
public int MethodInBase()
public class UnderTest : BaseClass
public int OtherMethod()
public void MockingSpecificMethod_AndExecutingTheRestNormally()
UnderTest target = new UnderTest();
using (RecordExpectations rec = new RecordExpectations())
As you can see only the call MethodInBase was mocked while all other calls will go to real code. By default this is the expected behavior for concrete classes.
:!: This behavior will change if you are mocking interfaces.
Anyway I hope this is what you meant. if not please post a short example and I'm sure we will manage to find a good answer for you.