There's no strict regimen for a casting session so you're off the hook there. Everyone has their own style and approach to what they are looking for. An experienced actor will grant you some latitude as long as you are professional and care about the project.
But that doesn't help you develop your own approach to a casting session. It is very common to have the actor read lines from the script, typically a page or two. Choose the most challenging lines for their role and send them the pages in advance (called "sides").
The actor should come with an interpretation of the character. However, it's always a good idea to talk to the actor beforehand and communicate about your ideas and listen to their ideas. This will help put them at ease. It's hard to go to a casting session and put yourself out there to be rejected.
This will also give you the opportunity to see how you work with them. Did they come prepared with an interpretation and ideas about the character? Do they listen to you? Can they incorporate your ideas? Are they difficult or arrogant? And, most importantly -- do they show up on time?
It is also common to ask actors to bring their own dramatic monologue. It should be something they are familiar with that they believe showcases their talent. Not necessary but you can provide that option and some will take advantage of that and some won't.
It is highly recommended that you videotape the rehearsal. People look different on camera than your in person impression - some better, some worse. Also, if you see a lot of people in a row then your memory can get hazy. Taping the performance is a must.
Bring the actor in, chat with them for a bit and then have them do a read or two. Each actor is unique. Some actors will nail it the first time and each subsequent read gets weaker and weaker. Other actors get stronger and stronger with each read. Give them some direction and see how they handle it. This is critical to establish whether or not you can work with them. Finding out if they are difficult now is far better than when you are on the set.
When they have done a few reads and give them a chance to do a dramatic monologue, thank them for their time and let them know when you will make a decision. Then, when you make a decision, let them know either way. It's hard to deliver bad news but it is the least you can do considering their effort.