What we are really after is figuring out what are the pure 'working relationships' an object has within its society. Each service an object is contracted to supply, or said differently - each role it plays, is embodied within an interface. So, designing using interfaces can be a mind liberating experience. With that in mind, let us put this theory into practice using an example. Say you have a domain where you need the ability to price some objects.
Some objects have no price, some have a fixed price, and some have a dynamic price.
The dynamic price may depend on various factors such as time and locale. This group of messages, this repertoire, has semantic meaning on its own, and as such it deserves to be a first class object. We call such reification - an interface. And the above repertoire defines the Priceable interface. Money or Product or WorkService or whatever' - we can refer to those entities as Priceables.