Kit Harington has spent more time than any other actor filming season 6 and his character’s actual name was not even mentioned in the scripts, call sheets, scene breakdown, prop/wardrobe materials etc.
Instead, Harington was given the secret code name ‘LC’ by producers, a short for ‘Lord Commander’. The cast and the crew were not “allowed to say ‘Jon Snow’ on set, ever, everyone had to refer to me as ‘LC,'” Harington told EW.
Only exception was when they were reading dialogues on-camera. Even showrunners David Benioff and Dan Weiss used the code name ‘LC’ when they were near outsiders.