If I create a Personal Hot Spot Wi-Fi (on an iPhone 8 with iOS 12.1.3), connect an Apple TV 4k to that Wi-Fi, will I incure mobile data usage if I airplay to the Apple TV using previously downloaded iTunes movies on the iPhone?
As you are not on a WiFi network, data will be used to transmit the signal.
If you have no wifi to connect to, then you will using cellular data which will eat up your data. don't want it to eat data, then get a wifi connection.
As you have the move downloaded the data consumption will be reduced but data will still be used.