What is a synonym for stand in?

Here are some synonyms for "stand in," depending on the context:

For a person:

* Substitute

* Replacement

* Understudy (for actors)

* Fill-in

* Temporary

* Proxy

* Delegate

For a thing:

* Surrogate

* Placeholder

* Alternative

* Proxy

For an action:

* Represent

* Deputize

* Act for

* Take the place of

* Step in

* Fill in for

The best synonym will depend on the specific situation you are describing.