The user should see the 'New Reply' form at the bottom of a Post's show page.
The user should be able to make replies using the inlined 'New Reply' form.
Error handling (trying to reply without any content) should still work the same way as when replying was a separate page.
What needs to happen in the posts controller to allow inline replying? What needs to happen in the view?
RailsGuides - Form Helpers, section 2.2: http://guides.rubyonrails.org/form_helpers.html#binding-a-form-to-an-object
Go on to Add Other Features Of Your Choosing