The Targaryens are known for their distinctive long, platinum hair, so obviously the producers wanted to see what the character would look like. D’Arcy currently (and at the time of the audition) has very short hair with buzzed sides.
Auditions for the series took place during the pandemic, which made an already fairly common audition technique — the self-tape — a necessity. Often actors will tape themselves on their phones or with a home camera and send it in for consideration. Before the pandemic, it was often a first step in the process, with in-person meetings after that, but as we all know, COVID changed the way a whole lot of things were done.
D’Arcy told EW that they had to come up with a “very speedy audition tape” for a series that they thought was a “Game of Thrones rip-off.” They recorded themselves on a phone (which the outlet says was “propped up against a bag of chips”) and sent it in. There were several more self-tapes, and then a “virtual conversation” with showrunners Ryan Condal and Miguel Sapochnik (who just announced that he is leaving the series). They asked if D’Arcy had a wig, and since stores were closed, the actor had to figure it out by themselves.