I can think of 3 Buffy episodes where he sang, and he was amazing. Always makes me think of Willow's comment, on watching him sing and play the guitar, that now she remembered why she'd had such a crush on him. And the way he could switch in a moment for Giles to Ripper, and it be so clear which was which. It takes a skilled actor to make you suspend disbelief, the way he did.