Demos are #1
One of the most important reasons that you want to hire a professional to produce your demo is that the professional you hire has years and years of industry experience, so they are going to know what people are looking for. They're going to really know what the current trends are in the industry and they're going to be able to craft a storyline to really help showcase your voice in the most positive light so that you can get hired from your demo.
Have you ever heard the phrase; "You don't know what you don't know"? Often times we think we know what makes a great Voiceover demo. You'll be tempted to make one on your own. But the reality is you really don't have the expertise to make a killer demo. Your VO demo has to be killer because you never get a second chance to make a first impression.
