General:
* Temporary
* Transitional
* Short-term
* Provisional
* Stopgap
* Acting
* In the meantime
* For the time being
More specific:
* For a period: During, meanwhile, pending
* Acting role: Substitute, replacement
* In the meantime: Pending, pending on, until, while
The best synonym depends on the specific situation and how you want to emphasize the meaning of "interim."