You should have a delay and response key. response key value is a predefined name from the responses.js file. You can alternatively use body or json key's to define text/html/xml data or json data. If you use something not predefined, set the response key to 'custom'.

Example: 
{
	"delay": 1500
	, "response": "custom"
	, "body": "<xml>custom xml </xml>"
}