"Do "creatives" kick off a render and then sit and wait for it to finish? Or do they do other work while the render process does its stuff?"
In general, no. The work would usually be done on either in-house render farms or third-party/cloud render farms, but all of this required a little planning/scheduling to get work done, particularly when busy and licences needed to be managed carefully.
However, if clients are coming in to review the work or a "small change of direction" is required, it was not unusual to run short jobs on local machines. Often, being able to quickly adjust something and show the client would make them realise why they were wrong and allow the work to be signed off in time for a few drinks versus making changes for the client to review later which often involved multiple revisions.
Biting the hand that feeds IT
