Thank you Elizabeth. I'm new to Rails so poking around in the Rails source is insightful.
Usually the test case is a good guide for how to use a piece of code. The interceptor is declared right inside the test case. The documentation and the test seem to imply that I should declare the interceptor and the registration in the same file and place it inside the initializers folder.
I'm implementing an email interceptor that redirects outbound email to a testing email account during development, so I added an interceptors folder under config and put the registration in development.rb. This seems a sensible place for interceptors to live. It also registers the interceptor only under the right circumstance.
Feedback about this is appreciated,