Ser ever switched to Estar?

[Anima]

Okay, I understand about ser and estar and the different uses for them. What I'm wondering is, how strict are those uses? For example, if someone was working a job as a secretary, but they knew from the beginning that the job was temporary, could they say "Estoy un secretario.", just to emphasize the fact? Or would that just be wrong? Or, another example, say someone was being mean and instead of saying, "Eres mi capitán," they said, "Estás mi capitán." Would that be at all correct, or am I thinking too hard?