I'm a total beginner here, so I apologize if this seems like a basic question. I'm trying to get the cast list after finding the id of a film. I can successfully get the original JSON info and extract the id, but when I try to call for the cast list, I get an error telling me I have an invalid API key. I have a key (which I use to get the original info), but it seems the documentation tells me how to get this with the "mock server info". My request shows up in the inspector, but I'm obviously missing something. My javascript as of now looks something like...

$(document).ready(function() {

  var getLists = function(){
    var filmA = $('#filmA').val();
    var filmB = $('#filmB').val();

    $('#return-list').html("<h2 class='loading'>Searching</h2>");

    $.getJSON("http://api.themoviedb.org/2.1/Movie.search/en/json/###/" + filmA + "?callback=?", function(json){
        var filmAid = json[0].id;
        console.log(json);
        console.log(filmAid);
        var xhr = new XMLHttpRequest();
        xhr.open("GET", "http://private-e886-themoviedb.apiary.io/3/movie/" + filmAid + "/casts");
        xhr.setRequestHeader("Accept", "application/json");
        xhr.onreadystatechange = function () {
          if (this.readyState == 4) {
            alert('Status: '+this.status+'\nHeaders: '+JSON.stringify(this.getAllResponseHeaders())+'\nBody: '+this.responseText);
          }
        };
        xhr.send(null);
      }
    );

    $.getJSON("http://api.themoviedb.org/2.1/Movie.search/en/json/###/" + filmB + "?callback=?", function(json){
        var filmBid = json[0].id;
        console.log(json);
        console.log(filmBid);
        $.getJSON()
      }
    );

  };

  $('#search').click(getLists);

});

Obviously, I'm ideally trying to get the cast list for two movies and compare them, but I'm hung up on just getting the cast list. Like I said, I'm sure this is a basic problem, but any input is greatly appreciated.

Thanks!