As I said, an interface can easily be tested, but if it's internal implementation I normally make a design, implement it, and I usually come up with other ideas and I like having the fluidity of making changes as I prototype and such, that way I'm not constrained by a test yet. After I'm done, and documented my choices if necessary, then I add the tests.

Except that then you'll shape your interfaces to drive tests nicely, instead of being, you know, sane and useful (been there, done that).