Temporary, provisional, ad interim, acting, supply can all be applied to a person holding a post for a limited time, to the post held by that person, or to his appointment.
Temporary merely implies that the post is not held on tenure but may be terminated at the will of those having the appointive power. It is interchangeable with many of the other words but is not so explicit.
Provisional is applied chiefly to a government or to the head or leading officials of a government that is set up in a new state or after a revolution until a permanent government can be established.
Ad interim definitely suggests appointment for an intervening period (as between the death or resignation of an incumbent and the appointment or election of his successor). The term is also applied to an appointment made by the president of the United States when the Senate is not in session and confirmation is not possible until after the recess.
Acting is applied to the person who during a vacancy in an office or during the absence of the incumbent assumes temporarily, by appointment or by fixed procedure, the powers given the person regularly appointed or elected.
Supply implies the performance of duties of another or service as a locum tenens.