Someone who has range, as you'll be reading two parts in the script. You must be able to do two different voices so listeners can distinguish between the parts. Perhaps by using a slightly different accent in each part. You must have a sense of humour. The script is dead-pan so it’s helpful to have a comedy actress for whom timing, tone etc. will come more naturally. Someone who can nail the subtle humour. For reference, someone who is like Katherine Parkinson or Kerry Godliman.
