When stuck on his pole, Charlie is an object, and you can cast spells targeted at objects on him. However, when removed from his pole, he is a living thing, and you can cast spells targetted at living things on him. You cannot cast offensive spells on him, though.
If you "squint at Charlie", he will tell you what the last spell cast on him looked like from his perspective.
You can also "gaze into ball", using Old Mother Dismass' crystal ball to see what the last spell looked like from an outsider's perspective.
Both of these features are useful when making tricks, letting the witch make sure they look right.
You can also use these features to see what non-trick spells look like to others, if you're curious.