Like Cassandra Clare's headcanon about Raphael in the books, his TV counterpart is also asexual.
The "I could have killed you"-"I would have let you" exchange between Jace and Simon was a direct quote from the book City of Ashes.
Valentine's line about the shape-shifting rune ("Shape-shifting, huh? Well, that's original.") was the writers being meta, referencing the exasperation of some viewers/readers with the rune.
Filming of the episode wrapped on November 23, 2016. This was the last episode shot, as part of the first half of the season, before the break from filming.