Dominic Sherwood (Jace), Katherine McNamara (Clary) and the rest of the cast got permanent tattoos of the angelic rune after wrapping up season 1 production.
Matthew Daddario (Alec) originally auditioned for the role of Jace. Upon seeing his audition, author Cassandra Clare told the casting directors he'd make a better Alec, and he got the role.
Stephen R. Hart played Brother Jeremiah in the movie The Mortal Instruments: City of Bones and has now been cast in the same role for the TV series Shadowhunters.
Harry Shum Jr and Matthew Daddario both auditioned for Magnus Bane and Alec Lightwood respectively in the film version of the TV series, City of Bones.
The whole cast of Shadowhunters are reading The Mortal Instruments series so they can better understand their characters.