1) Interceptors are not actions, and are not treated as such: request parameters are not set on interceptors, and doing so would be a potential nightmare. Ideally, interceptors are completely ignorant of other interceptors in the interceptor stacks.
2) This is an inappropriate use of an interceptor: interceptors are designed to handle cross-cutting application concerns, not logging someone on. Logging someone on should be handled by an action. *Checking* if someone is logged on would be handled by an interceptor.