Any time that the cast of the TV series gets to show that they're not only talented actors, but also great singers.
At the end of the Historical Fashion Fix for the Celtic warrior, the warrior isn't satisfied with his makeover and works himself into a battle frenzy. You expect the host to get chased off-screen in a comedic manner, but instead he stops the warrior, just like that.
Host: Not. On my show. *sassy fingersnap* Sister.
On the Rosa Parks Civil Rights song, props to the main cast for playing the part of the white bus passengers instead of leaving it to extras, though it must have been supremely uncomfortable to do so, considering the subject matter.
Alexander The Great. At the age of 20, he lost his father to assassins. How did he react? He ruthlessly crushed the rebels, and then moved on to conquer Persia AND Egypt, all in the span of 5 years.