Mic Runners are a great introductory way into theatre tech and are primarily responsible for looking after the radio mic(rophone) packs and headsets. Usually based around the mic table before the show - in the ADC Theatre this is generally between the dressing rooms -, mic runners will help the Sound Designer in turning on and handing out mic packs and checking that headsets have been put on correctly. Also a guardian of mic tape, mic runners may help actors with actually putting on the headsets and packs. During the show, mic runners will be on hand in a backstage area to replace batteries in packs if needed and also communicate 'on the ground' information to the sound designer if there are problems.
It is a very low commitment role since runners are clearly only needed during on show nights themselves and it is common for people to only a do a few nights of the run. As an assistant to the sound designer, however, there is always opportunity to help with the wider sound set up in the get-in, particularly in larger musical productions such as LTM or Panto.