Whether you're a casting director auditioning dozens of actors for a critical role, or an actor trying to nail down the rhythm of your lines, it is important to consider your "beats." What are beats?
They're basically the length of time in between spoken lines in a production. Think about it, the length of time between sentences we speak varies according to the subject matter and emotional import of the conversations we are having with another characters. An actor confessing that they've just been diagnosed with cancer is going to speak slow and deliberately, and may even get choked up in between sentences. Whereas an actor getting angry after discovering an infidelity is going to talk rapidly and excitedly.
Because the beats of these types conversations take on familiar and recognizable patters, it's important for casting directors and actors to be mindful of them during the auditioning and casting process.
Actors: Don't just memorize your lines, and work on inflections- try to think about the exact way you'd deliver a life in real life. Try to imagine all of the peripheral things that would be running through your head as you lived through a particular scenario.
Casting Directors: When casting for a role, try to put yourself in the scene- try to imagine that the actor is speaking to you directly, and that you are being immediately impacted by the words emanating from the actor's mouth.
If we all pay attention to the beats of a scene, the scene will play out more naturally, and your performances will ring more true.