I could write a test plan first, but I haven't always fully designed the interfaces until I've ploughed into the code and figured out what needs to be passed wh

I could write a test plan first, but I haven't always fully designed the interfaces until I've ploughed into the code and figured out what needs to be passed where, so there would be a lot of repeat effort in fixing up the tests afterwards. Effectively, in writing tests first you make assumptions about the code. These don't always turn out to be true.