One of the first decisions you have to make when choosing voice talent online is whether you need a male or female voice. Needless to say, the different sexes can provide a completely different sort of appeal for your content. You may want to initially audition both male voice talent and female voice talent to consider which works best for your project.
However, once you've established that, there are still a lot of options to consider when casting your voice female or male voice talent. Consider the setting of the story being related. The geography of the story might lead you to choose an actor who can provide or naturally has a certain type of accent. The subject matter of the story is also important. A story about a timid young man wouldn't make much sense read by a male voice talent with a gruff, commanding voice.
Remember, the most important decision is the one you've already made: to use professional voice talent!
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The type of professional voice talent you choose is just as important for non-narrative voice work. In advertising, for instance, you need a voice that fits the attitude of your product or service. If you're working on a political ad, you need a voice with authority, confidence and believability. If you're advertising a television adventure series or video game, you might need that harsh, dry voice that implies danger and excitement. Or, for more generic products, you can find someone with a nice glib and friendly salesman-like voice. Keep your audience in mind, and you can find excellent voice talent online for any project.