Halliwell, formerly known as Ginger Spice, tried several times to recite the simple phrase, but couldn't really make it believable. Harington, in turn, would repeat it back with the gravitas the line required.
The line is of particular significance to Harington, and not just because it is directed at his beloved character.
"You know nothing, Jon Snow," was a line spoken numerous times by the would-be King of the North's Wildling love interest, Ygritte (played by Harington's real-life girlfriend, Rose Leslie). The line became a kind flirtatious banter between the pair (and eventually, a heartbreaking last exchange).
It only makes sense that Ginger Spice would try to take on the mantle of one of GoT's most beloved ginger characters, but it doesn’t look like fans are likely to see any Ed Sheeran-esque cameos from Halliwell anytime in the final season.