I haven’t acquired the fine taste for TDD, but I’m with you on 1 and 3. In my experience if the interface isn’t already there, I’m not as likely to even try to make a second implementer. And yes, “subclass and override” is great when you need to build some momentum without breaking the whole system, but of course you’d rather have an interface if you can swing it. It’s bound to require less typing and thinking than figuring out what you should and can override in a subclass. Stub or mock the whole thing and be done with it!