A random witness (amongst the witnesses with the most votes but not within the top 21) is also chosen each turn to produce a block, these not top 21 witnesses also take part in the votes for a hardfork. So while they are mainly backups, they still participate in signing blocks from time to time.
Witnesses are not miners but miners are witnesses. As a miner, when you found a block, you get a ticket to become a witness for one block/one turn.
RE: [Steemit Update] You can now vote on witnesses who are outside the TOP 50