I disagree with the assumption that you can't sing from a musical if you are going in for that show. I think though, that if you are going it with a certain character in mind and you are singing that material, you have to be pretty darn good. You're saying "Here I am. I'm your wo/man for the part." If you aren't prepared to be that character at the moment, I would say that singing something of similar style and sensibility is always a good choice, no matter what the material.
Trevor
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