A councillor who is acting as proper officer retains the full role of councillor. That is one reason why it is not an ideal situation, although it may be helpful as a means of filling a gap. It is difficult to take part in dicussions and votes at the same time as recording minutes and monitoring proper observance of the law.
Remember that the councillor must be properly appointed by a council decision, and the councillor may not be involved in the discussion or vote, since the clerk is a position held for reward, even in the case of a councillor who does not in fact receive any pay.