acting ... temporarily for(просто acting, но если надо указать за кого, то temporarily for • The 25th Amendment allows the vice president to serve as acting president temporarily in the case that the president is ill or otherwise temporarily unable to fulfill his or her official duties.Farrukh2012); locum(временно исполняющий обязанности • He's only got to find a locum for two weeks.Aiduza)
alternate(временно исполняющий обязанности отсутствующего лица BrinyMarlin); designate(designate is used after the title of a particular job, esp. an official one, to refer to the person who has been chosen to do that job, but has not yet started doing it. E.g. the Governor/Prime Minister/Secretary General designate. andrei+++)
