Hi,
Not sure understand since you refer to "MyVariable" as constant string and like a variable.
Here is my best shot :)
You can use sequenced fakes, that means that when you use WillReturn more that once the first time the method will be called it will return the first value and the second the second value you set and so on.
Example:
[Test]
public void Test()
{
DataAccess fake = Isolate.Fake.Instance<DataAccess>();
Isolate.WhenCalled(() => fake.ExecStoredProc(null, null, false, false, false)).WillReturn("My faked string");
Isolate.WhenCalled(() => fake.ExecStoredProc(null, null, false, false, false)).WillReturn("My other faked string");
Assert.AreEqual("My faked string", fake.ExecStoredProc(null, null, false, false, false));
Assert.AreEqual("My other faked string", fake.ExecStoredProc(null,null, false, false, false));
}
Hope it helps.