Whilst it's nice and easy to create JSON services for your front-end JavaScript if you're working in ASP.Net MVC, things can be a bit more difficult to wire together if you find yourself with project requirements that force you to work with the older bits of ASP.Net. Getting the JSON inputs and outputs to work can caused me some head-scratching.
Having had to make this work recently, I thought I'd write down where I ended up after searching Stack Overflow and MSDN...