You must be able to know how to put audio files together and turn them into a complete conversation. You must know how to make a difference for when a character is thinking and when a character is talking out loud. You must know how to change the sound depending on where they are for example; if they are in a cave then their voice should echo. You must know how to do/add sound effects and background noises without making it sound unnatural. Lastly, you need to know how to make words fade into the screen when the voice actor starts talking (You can look at episode 5 to help you understand what I mean).