Beyond Java book had a good example of Interceptor in Ruby. It is like AOP, but is simpler.
1. Create an alias for the method that you want to intercept.
2. Define a new method with the same name as the old method
3. Delegate the call to the old method and add new code to do whatever you want in this method.
You basically open the class, intercept the calls to that method and add additional functionality to the method.