"On duty" and "on guard" are common phrases that can be used to refer to someone who is performing the duties of a designated person, such as a guard or a receptionist, during a particular shift or period of time. For example, you might say:

"I'm on duty tonight, so I can't go out."
"Who's on guard this weekend?"

Another common phrase that is often used to describe a person who is responsible for certain tasks or duties during a specific period of time is "duty officer." For example:

"The duty officer will be available to answer any questions or concerns you may have."
"I'm the duty officer for the next eight hours."

In some contexts, you might also use the phrase "roving reporter" or "roving correspondent" to refer to someone who is responsible for gathering and reporting news or information from different locations.

I hope this helps! Let me know if you have any other questions.