act up

verb

1. To misbehave; to cause trouble.

   “My allergies act up during hay fever season.”

   “Children may act up in class in an effort to get attention.”

2. To perform the duties of an appointment senior to that actually held; to deputise for a superior.

   “Acting up is always recommended when you find yourself under a new boss.”

   “A few months through my contract, my clinical supervisor said that a registrar was leaving the post early for a research job, and asked if I’d be interested in acting up on the rota.”