Taking on an ‘acting’ role can be a great way to try out a new role with the safety of knowing it is for a finite amount of time. If you are thinking about moving into leadership, then it can be a perfect way to dip your toe.
The thing about acting positions is that their timing is rarely predictable. In my career, I have acted up twice and gone on secondment twice. The first time, my phase leader damaged her knee and was unable to walk for a term. The most recent time, my head gained a much-deserved role as an SIP (school improvement partner) in a neighbouring county. I was tasked by the governors to take the helm while an experienced replacement could be found.
The main differences between a secondment and acting up are that with a secondment you are able to choose whether to apply. It will be in a different school and usually takes the form of a fixed term contract. With acting up, you may not have a choice! If you are in school leadership and your line manager is absent for any reason, then tag - you’re it!