Effective communication in a sysadmin context is about the usual things, but some areas are more important:
- Communicating technical concepts to technical peers and non-technical staff.
- Providing a pleasant desk-side manner in the case of troubleshooting (that may be the job of the helpdesk most days, but we get called out every so often anyway).
- Ability to talk to both project-management and line-management in ways that are beneficial to both.
- Able to write documentation that is useful to more than just the writer.
But your question was how to address this item in an interview. Personally, I've used my ServerFault profile and a few articles I've managed to get published to help address this very question. If you have anything like that out there on the Internet, even if it was just a casual forum somewhere where you went deeply technical and it helped, that kind of thing is valuable: it allows the Interviewer to see a sample of your work.
If you don't have enough of an online presence to field this question completely, keep in mind that we all do communication every day. And these days, most of it is recorded. If you've ever had to shepherd a very expensive project-proposal through a budget process, you've effectively communicated and can put that on a resume. If you've received praise on the quality of the documentation you produced, that's resume fodder as well.
When I'm looking for a sysadmin, the communication-skills I'm looking for are:
- Not likely to turn into the snarly kind of sysadmin people don't like to talk to (an unapproachable sysadmin is foregoing the human layer of the monitoring framework).
- Shows some proficiency with writing down technical concepts for later re-use (also known as "documentation").
- Ability to explain highly technical concepts to less highly technical people.
- If the position demands it, ability to explain why a potentially big change is a good idea and we should do it.
Some of this can be assessed by an interviewer through just watching the interviewee react to the whole process. Body language, word choices, who they pick to talk to, are all useful details. For the rest, I'd like to see examples if possible, but failing that I want stories. Stories of budget battles won. Stories of post-mortem sessions after a big failure of some kind. Stories of Big Problems being troubleshot through and how the interviewee handled status updates. Stories of unclued coworkers becoming clued through the interviewee's actions.
All of these are useful and highlight different facets of the communications skills I'm looking for. They're also, not surprisingly, the kind of stories we tell each other over beverages at conferences. Focus less on the technical issues and more on the communications aspects, and you'll have a good stock of answers for these questions.