If you want to run an interactive login session by logging in to a system using a terminal that is connected to a serial port, and then using the consadm command to see the console messages from the terminal, note the following behavior:
If you use the terminal for an interactive login session while the auxiliary console is active, the console messages are sent to the /dev/sysmsg or /dev/msglog devices.
While you issue commands on the terminal, input goes to your interactive session and not to the default console (/dev/console).
If you run the init command to change run levels, the remote console software kills your interactive session and runs the sulogin program. At this point, input is accepted only from the terminal and is treated like it's coming from a console device. This allows you to enter your password to the sulogin program as described in Using Auxiliary Console Messaging During Run Level Transitions.
Then, if you enter the correct password on the (auxiliary) terminal, the auxiliary console runs an interactive sulogin session, locks out the default console and any competing auxiliary console. This means the terminal essentially functions as the system console.
From here you can change to run level 3 or go to another run level. If you change run levels, sulogin runs again on all console devices. If you exit or specify that the system should come up to run level 3, then all auxiliary consoles lose their ability to provide input. They revert to being display devices for console messages.
As the system is coming up, you must provide information to rc scripts on the default console device. After the system comes back up, the login program runs on the serial ports and you can log back into another interactive session. If you've designated the device to be an auxiliary console, you will continue to get console messages on your terminal, but all input from the terminal goes to your interactive session.