The Interact API is a RESTful JSON API. This means that data is returned in JSON format, and you therefore should always set an 'Accept: application/json' header on all requests. Where a method is a POST, data being submitted should be included in the body of the request and the content type should be specified as 'application/x-www-form-urlencoded'
Additionally, all requests should have a header called X-Tenant which specifies your Interact tenant GUID.
Errors are usually returned in JSON format, though you may get a text/plain response in some cases of an error. You should treat this as an error you need to fix.