Usually what happens when a party is not in power is that the leader will resign and an interim leader will be appointed until a new leader is elected through a

Usually what happens when a party is not in power is that the leader will resign and an interim leader will be appointed until a new leader is elected through a leadership election. When a party is in power however, that interim leader would be an "interim Prime Minister" and is avoided because it's a lot of responsibility to give to an interim with no mandate from voters or the party.