Facial expressions, body posture and tail movement all indicate a cat’s mood. To determine what your cat is thinking you should observe their body language as a whole not just isolate one aspect of it.
A relaxed contented cat points their ears forward, half closes their eyes, curls the tail and purrs. A cat that is afraid will have their ears back folded flat on the head, and their pupils will be dilated. An agitated cat has completely dilated pupils, flattened ears, forward sweeping whiskas, and may even show his/her teeth. A defensive cat will make themselves as big as possible, including brushing their tail out and extending their legs and backbone. And, if you see a quickly twitching tail, this indicates excitement or imminent aggression.