Follow the Casting Director's Instructions
Casting Directors will often provide detailed instructions on what they need from you.
This can include how many takes are needed, file naming requirements, and what they are seeking in the performance. Be sure to read and follow the provided instructions. Failing to follow instructions can hurt your chances.
Shoot in Landscape Mode
Here is an article that will help you correctly orient your device when shooting.
Shooting the video in landscape (horizontal) mode helps prevent the video from appearing sideways or too small when viewed by the Casting Director. Avoid shooting in portrait (vertical) mode.
Use a Backdrop
Ideally, you should perform in front of a solid colored curtain because the waves in the fabric provide depth. A good second option is to find a blank wall.
Too much clutter in the background can distract from your performance.
Also, make sure that you wear clothing that does not blend in with the color of your backdrop.
Try to record in a quiet room with a lot of soft surfaces to reduce echo/reverb.
Rooms with a lot of echo can make your dialogue sound muddy and hard to understand.
While the camera mic is acceptable in most cases, it can help to use either a shotgun mic or lavalier (small lapel) mic to record your dialogue. These will help reject background sounds so your voice comes through clearly.
Make sure you’re well lit from a light source(s) in front of you. Daylight from a window is best for even, natural lighting. Or, inexpensive LED camera lights are available from a variety of retail sources. Otherwise, position a bright, bare lamp or two to the left or right side of the camera for shooting your audition.
Be aware that lights directly overhead create deep shadows on your face which can make you look older or change your appearance completely. Also, fluorescent lights tend to make things look more yellowish-orange.
Avoid bright lights or stark white backgrounds directly behind your head.These can overpower your front lighting and make it harder to see your face.
Find a Quality Reader
One of the key things Casting is evaluating is your ability to deliver and react to dialogue in a believable manner within the scene. Therefore, (unless otherwise instructed) do not simply record just your side of the conversation.
Find a friend who can stand off camera and read as the other role(s) in the scene. The reader doesn’t need to give a full performance, they just need to say the lines clearly.
Make sure your reader stands behind the camera, or at least directly next to it, to ensure that you are performing towards the camera so we can see your face. Unless specifically requested by Casting, never show your reader on camera with you.
Unless specifically requested by Casting, make eye contact with the reader and not the camera lens.