You should have a delay and response key. Response key value is a predefined name from the lib/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'.
The object should be a valid JSON. In case of sending text, xml or html data in the body, make sure the content is passed as a string and is properly escaped.

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