During the 'Old Friends' scene, converse however you wish.
The Norah part is clearly much longer. To get inside the hospital, first ask the
employee behind the desk, then take control of him with Aiden and hit the
elevator switch. To get past the guard, hit the switch. Hmm, since it isn't
working, first distract the guard by interacting with objects and the
electricity box. To get past the next door, again interact with objects. Be sure
to hit the final chairs and table against the glass so the employee wakes up.
As het gets closer, take control of him and open the door. Grab his keycard
(on top of the card reader next to the door) and continue.
You can hit the cameras with Aiden; in any case be certain to pay a visit to
room 38 (with a humming man inside) to collect the [BLUE ORB] with Aiden.
Norah is in room 43. To keep spoilers to a minimum, this guide doesn't describe
anything regarding the next scenes, but they are very easily accomplished either
way. The final option is up to you. Just keep in mind that it affects a trophy
with regards to how many characters you have kept alive.