In addition Tertullian made it clear that discipline needed to be observed in the appointment of people to the various ministries.

And so it comes to pass that to-day one man is their bishop, to-morrow another; to-day he is a deacon who to-morrow is a reader; to-day he is a presbyter who tomorrow is a layman. For even on laymen do they impose the functions of priesthood.

The nearest text we get to explaining the job-description states:

let the reader stand upon some high place: let him read the books of Moses, of Joshua the son of Nun, of the Judges, and of the Kings and of the Chronicles, and those written after the return from the captivity; and besides these, the books of Job and of Solomon, and of the sixteen prophets. But when there have been two lessons severally read, let some other person sing the hymns of David, and let the people join at the conclusions of the verses. Afterwards let our Acts be read, and the Epistles of Paul our fellow-worker, which he sent to the churches under the conduct of the Holy Spirit; and afterwards let a deacon or a presbyter read the Gospels