Funny thing about dialogue: you need two people to have one. That means if you're shooting a conversation of any kind, you're going to have to cast more than one person.
This sounds terribly remedial, I know, but it's important.
So let's say you're running a casting session for a scene that requires dialogue and you have two actors come into the room together. They slate –– one at a time, of course –– and then they go through the action.
Which one are you looking at?
See where I'm headed with this?
If one of them sucks and the other one's great, what do you do?
I'll tell you what you do. You go back in time. Before you even begin working with your actors, you take your video camera and point it at one of them –– doesn't matter which. Frame the other person completely out. Then as you're watching the performance, you make notes only about that one. And since you're a conscientious person and don't want to give one of the actors the impression that you're ignoring him, you tell him that once you're done working with the one guy, you're going to concentrate on him.
When you've got the first person's performance to a place you like, tell your first actor to keep things right there and turn the camera on the second actor. Then work with him.
There are a couple of benefits to doing things this way. First, you only have to keep your mind on one performance at a time. More critical, if you're like most directors you don't get the final say on casting. Other people are going to have to discuss your choices, and they can't discuss what they don't see.
Which brings me to a rule I try to apply, not just with casting, but with every aspect of production: Never submit anybody or anything you wouldn't be 100% happy to work with. I guarantee you that if you give somebody a choice, they're going to choose the one you don't like.