Hi.
Using the witness blockchain technology, they produce Steem blocks, which means they do Steem mining in a way.

They follow and expand the economy of the Steemit platform. They try to protect this.
I mean the witnesses are the people who manage the Steemit server part. All data of the users are kept in these servers.

If I come to your question. Yes, you can be witness without doing activities on the steemit. But of course not everyone can be witnesses. It must have technical skills and solid leadership behind it. It is advantageous to be familiar with steemians. So there may not be any activity here, but I can say that witnesses are very laborious. Secret hero. Especially @steemchiller, he's a superhero in my eyes, thank you. :)