I think the main requirement for an exorcist would be a very close, deep & comfortable/non-dramatic & very grounded relationship with God.
A priest under stress or pressures or who sort of has done more redundant chanting as a substitute for quiet, calm deep faith would be at great risk.
I saw a drama-documentary once on an Italian priest who specialized in exorcisms. He supposedly was a real guy though. His technique was very nonchalant. He’d done so many exorcisms that he knew the demon’s tricks. He taught other priests how to do it. Possessed people would foam at the mouth & scream & the priest was all like “yeah, whatever”...*splash*. On went the holy water. The subject would writhe in agony. I think at one point he even had to take a call on his cellphone & walked out into the hall for a minute.
What you’ve mentioned in the top paragraph is part of what I’m talking about. So isn’t the relationship with God something that is more or less constant? What I’m trying to understand is what type of preparation is required and wouldn’t it be something ongoing anyway?