In the story, the characters using magic knows it's dependent on the old gods, and those gods are not always cooperative. There is no sure way to make something happen with magic, but intent and confidence seem to matter, even though Merlin more than implies that a lot of it is also showmanship.

I love this depiction because it's chaotic and unpredictable for everyone involved, and the results are never guaranteed. There is no mechanical way to ensure a wanted outcome, and the people using magic have to work freaking HARD to get things to happen.