((Hello, I'm Secret's sister and this is a different kind of RP then usual. It can either be easier or more confusing then the normal RP and that depends on the people involved.
The way this kind of RP works is the first person posts an intro. Simple right? Yes, but then it can get confusing. Anything the starter makes direct reference to exists (like if their character sits down in a chair. Therefore chairs exist) anything else is Schrodinger's cat both in the state of existance and non existance and it's up to all the other RPers whether things exist. There are also exceptions to the rule (One person establishes cats can't talk. The next person has their cat character talk which is an exception. The other characters should be surprised because it has already been stated that cats can't talk). This allows people to RP who or whatever they want.
So the first person says: "Sophie collapsed into her spotted blue couch and petted her tabby cat gently. The cat purred in response." This posts states a plethora of things. There are couches. There are cats. The cat has a tabby coloration. The couch is blue. Sophie is a girl. The cat can purr. There are also several things that are implied; The cat likes getting petted. Sophie has hands to pet the cat. Sophie is huminoid. The cat has four legs, a tail, fur, etc. Even though those things weren't specifically mentioned we can guess they are true unless someone else says otherwise.
The next person might say something along these lines: "Victoria entered the room and scowled down at the girl on the couch. She set her coffee mug down on the table in front of it and slumped back into a chair. She gently called the Rex, the brown tabby cat, trying to coax it over so that she could pet it." This person defined many things in the world, but they did make one mistake. Since the first person didn't mention whether or not they wanted the cat to be their character or an NPC the second person should have asked to make sure it was alright to name the cat.
Wow, I talked for a long time. Sorry about that. I'm just going to say one last thing: If you introduce a new character say in OOC at the bottom of your post whether they are your character or an NPC. If it is an NPC then anyone can say things about it or even claim it as their own character if they want to.))
Nat lay with his nose in the grass, watching the children play. He had been asked to keep an eye on them, though he was sure the mothers meant it jokingly. Who would trust him with any sort of responsibility. Everyone always commented on how lazy he was. All he ever seemed to do was lay in the sun watching people pass. Of course he did more then that, but people never saw him. He was the protector and just because there hadn't been great need recently didn't mean he was useless.
A sound near the tree line caused him to jump to his feet, prepared to defend the children if he needed to. His long golden tail swished against the low grass and he was more then prepared to act if need be. His nose twitched a bit, but he knew that he wouldn't be able to smell anything. Maybe if he had been a dog, but this silly feline nose could hardly smell anything. ((Nat: My character. A long haired golden tabby cat.
Children: NPC
Mothers: NPC
Noisemaker: NPC))