Every element of your argument should be supported by strong reasoning and/or factual evidence (that is in part what makes for a complete argument). You may include information found outside of the case, but only if it is fairly basic/widely known. You should not engage in substantial outside research. The case is far richer in detail than it seems at first glance and it is quite possible to make a complete argument while relying exclusively on information found in the case combined with information you can derive from the information found in the case (that last part is a very important component of developing strong problem solving skills).
Your paper should be formatted in the 7th edition APA style and include references as required.