It really depends greatly on the balance of the team as to what role is required of the lock. Basically they are just another second rower so they are expected to do a lot of work defensively and generally run wider off the first receiver as opposed to a front rower who would hit it up back through the middle straight off the ruck. Often the lock possess great ball skills and can be used almost a second 5/8th, like Glen Stewart or Feliti Mateo. Often they can be the defensive workhorses of the team, like Dallas Johnston or Corey Parker. Sometimes they might be used as a damaging ball runner or enforcer, like Paul Gallen or Adam Blair. As for where they stand, in attack like I said they usually run a bit wider off the halfback or 5/8th. In defence they are usually towards the middle, regularly topping the tackle count. From the scrum they pack down at the back, with their head between the two second rowers.
