Ultimately, it's down to skill. 'Ratt-tatt-tatting' on a door will always sound richer and more interesting than knocking. But it takes an ear for it to make it fit. James Joyce was a genius to be able to make up words which mesh together on such a consistent basis. Insane, but still ingenious. Also, neologisms (such as 'Apparating' from Harry Potter) is different from just making up words. Neologisms are born out of necessity -- you have something new and unique which needs a name of its own (i.e. there isn't a word for using magic to instantly travel somewhere)