Which objects i should mock?

I have a method say
funcA( Object1 1, Object2 2)
{
1.x = GetSum(List<Objects>,x);

2.y = 100 * 10;

//some more processing on these objects
}

Then, when should I create Isolate.Fake.Instance<Object1>() or use the actual businee object like object1 obj = new object1(); obj.x = 10....and set values for the other objects as well.