There are voiceovers who can sight-read a script and instantly work out what is required. There are others who have to be directed word by word. Many scriptwriters have not realised this. If you're reading this because you want to be a voiceover, look at this USA page click here . Click here .
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For more useful links about all sorts of things click here . They can totally ruin your script - or they can turn your mediocre script into something vaguely acceptable. If you've got a friend who is an out of work actor - steer clear unless you've got the time to train them. Being a good voiceover is only partly about the quality of the voice. It's also about general skill levels developed through experience and application.