Also, it's all too easy to construct questions that have not been asked before, which a human could easily answer but a machine like you describe cannot.
I also question exactly how unique individual elements of our lives are. In aggregate, they are probably fairly unique, but at an elemental level, we're all probably fairly mundane.
It won't be as fast as a real human, but with some clever stalling techniques, it might be able to pull it of realistically.
In reality, if someone pulled a program like this off, it wouldn't just be Sam and Samantha. It would be thousands of different personalities, trying to build a realistic social network between its own personalities and between its human users. It would be like a mirror held up to society itself.