First introduced disguised as a servant, Iwan Rheon quickly become every bit as sick and twisted a villain as Joffrey over the course of his four seasons on the show, rising from being a bit-part player in Theon's storyline to one of the primary antagonists.
Rheon did an incredible job of showing the demented glee Ramsay took in terrorising his victims, so much so it's hard to imagine him playing someone so completely different, but the actor did audition for Jon Snow - a man of honour who takes no glee in anything.
He explained to Interview Magazine that he did a general audition, reading the part of Viserys, before going back to specifically apply for the role of Jon. He agrees, though, that the casting directors made the right choice, and it's impossible to argue. Kit Harington is a better fit for the more stoic role, and Rheon is a delight as one of the most wicked villains Westeros has seen.