i looked up the definition of acting in the legal tense and put in simple terms if you perform the powers and duties of an office you are acting in the capacity stated if senator bruno is performing the powers and duties of lieutenant governor without holding the actual office of lieutenant governor makes him even without the formal reference the de facto acting lieutenant governor it'd make sense that an acting lieutenant governor would only become acting governor if need be for your reference here is the wikipedia article on the legal use of the term acting