Most available interims don't have the time to assume all, or most, of the previous pastor's duties. Generally, what I've observed is that the interim preaches, and provides counsel to the church's volunteer leadership on big decisions. Most of them work full time at something else. The salary is generally more of a speaking honorarium than it is salary for a job that is performed. Half of the previous pastor's salary is far to much for that. But that's become the custom.
At a church where I was once a member, but not on staff, a recommendation was made to engage the services of an adminstrator at a well known university as interim, requiring a round trip plane ticket each week, hotel expense, plus the salary, which altogether was a dollar amount nearly equal to the previous pastor's salary. In the business meeting where the recommendation was made, there was a lot of objection, given that the church was not meeting its budget and was slow to pay off some debt. The remaining staff would wind up doing most of the pastoral duties anyway, and they were capable of preaching but the personnel committee insisted that the church needed a preacher of the "calibre" of the university administrator in order to weather the interim period. Over 40% of the church voted against the recommendation, but they pursued it anyway. I'm just not convinced that someone in a position like the executive director of a state convention should put themselves in a position to serve as an interim pastor.