What is your cat trying to tell you?
Cats can have a mind of their own however over time any cat owner will become an expert in the body language of cats. You soon learn to recognize the signs when you cat is ready to play and is calling you to join him or her or when they are very content and ready for a nap.
A waging tail signal a conflict while the tail held high will signal confidence. Slowly blinking eyes will signal relaxation, while when your cat turns over on her back and exposes stomach it means he or she is happy and relaxed. On the other hand when they turn on their back and have their paws at ready they are getting ready to defend themselves.
Although there is no general guidebook, for example a rigid body and a fixed gaze might indicate the cat is getting ready to attack or the fixed gaze might mean your cat just wants a cuddle. Slit pupils can mean that cat is in almost vegetative state or it might mean cat is feeling predatory. However when it comes to eyes - fully dilated pupils are ok in the evenings, however in full daylight or in a well lit room they might mean that the cat is in pain or getting ready for fight or flight response.
Eyelids also tell a story, in general fully opened eyes means that the cat is ready for action, while the semi closed eyes mean the cat is in a relaxed state or getting ready for a nap. And if they still look at you and flutter their eyes lids slowly it is a great compliment for any cat owner, because this means they trust you enough to have a nap while you are present.
The mouth are not so indicative, unless your cat is bearing his teeth and hissing, they are scared and intimidated and might attack. My favorite by far is when cats gape, their jaw drops open and it looks like they are trying to vocalize sound "o". It is pretty cute, but they are only sniffing the pheromones that are coming from somewhere.
Body position will tell you a lot, although there are basic two you will have to learn to recognize - the offensive and the defensive.
On the defensive - your cat will lean close to the floor and move backwards away from the perceived source of threat, the head will in all probability go sideways and sometimes the cat will hiss, growl or emit some other sound. Also the hair will go up that is to make appear more bigger and scary looking, although the cat in this position will rarely attack as he or she is scared.
On the offensive - your cat will lower his or her head and move directly to the source with eyes fixed and moving from side to side while the ears will go sideways as well and the body will be shaped as a wedge. The hind legs will stiffen. Cat in this position is ready to attack, so beware.