Acting - temporarily doing the duties of another person. "acting director"

Interim - used or accepted for a limited time : not permanent . "interim coach"

The modifier "acting" applies when the executive post is filled on a permanent basis, yet the permanent appointee is unavailable, e.g. because of extended travel, sabbatical leave, or illness.

The modifier "interim" is used when the executive post is vacant. In other words, an acting officer serves in the absence of a permanent appointee, but an interim officer serves through a period between permanent appointees.

http://ask.metafilter.com/68674/any-difference-between-acting-and-interim-in-job-title