You need to choose scenes for the actors to read that have some transitions in them. Usually scenes with stunts, nudity, or emotional breakdowns aren't used for the first reading. Sometimes scenes from other scripts are used if they are better suited for an audition. The Screen Actors Guild requires that these scenes (sides) be made available to actors a certain length ahead of time before their auditions.
It is a good idea to have someone uninvolved with the casting read with the actor during the audition, if one is needed. One of the most important things you look for when casting is actors who listen, so you need to evaluate how they play off of their partner in the scene. This is why bringing in an actor to do the other side of the scenes is helpful.
What you should be looking for is the actor's choices, not a performance. Noting the creativeness and specificity of the choices, as well as how he responds to the text, will give you information about how he would approach the role. Also, seeing how they respond to your directions and adjustments will be important to gauging how they will respond to the same during work.
