The phrase "acting leader" refers to a person temporarily taking charge or leading a group or organization for a specific period of time, usually when the actual leader is unavailable or temporarily absent. Full definition
However there would have been a problem if she had appointed as acting leader and then decided to run as deputy to one of the candidates leaving in due course the position. (leftfutures.org)
But the party's deputy leader, treasury spokesman and now acting leader has proved unexpectedly adept at the job. (news.bbc.co.uk)
Be prepared to take part or have a go at any exercise, but don't try to act the leader if it doesn't come naturally. (allaboutcareers.com)