Multiple user logins from the command line

1. Can you start OS X window based applications from the command line of a non-logged in user (i.e., a user not logged in via the GUI)? 2. Is there any way to su from one user to another without being prompted for a password - the user does not have a password set? 3. Can you rlogin locally from one user to another and utilise .rhosts or .klogin thus emulating what is required in question two?