I just went through the same questions last week and here is my understanding :
you need to have ClientLoginModule (from JBoss) in addition to any of your own login modules (in the JAAS configuration). When you do login(), your login modules + the JBoss ClientLoginModule are then executed. The execution of ClientLoginModule binds user identity and credentials to JBoss, so afterwards they will be automatically passed in calls to JBoss. You do not explicitly set them in InitialContext or anywhere else.
(As I am also quite new to this model I welcome any corrections)