An example of Joss Whedon’s excellent writing style. His trademark is leading is audience into expecting one thing and then surprising them with an extremely creative twist.
In this case, Buffy and Angel fool Faith. Angel pretends to revert to his alternate personality Angelus, but it instead is revealed that he was acting the entire time. The audience reacts in the same way that Faith does - though it might be a different “Oh shit!” than Faith’s