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, as well as 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 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 a color that neither blends into nor contrasts with the color of your backdrop.

Avoid overhead fluorescent lights:

If it’s all you have, you can make it work with overhead lights. But be aware that overhead lights 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.

Find a quality reader:

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. Never, unless Casting specifically requests it, show your reader on camera with you.