Likely unbeknownst to many, you can manually choose which worker should fill a job opening, as long as you have zero free labourers that could (and would) auto-assign themselves. The plus sign quickly gets replaced automatically by the portrait of the new employee if someone is available, but if not, you can click it instead and bring up a new panel. There you can sort your people by name, current profession or distance to the chosen workplace.
Please make this panel available all the time! Just add a magnifying glass symbol next to the portrait of the automatically assigned worker to find them, leaving clicking their portrait to opening this panel. Or else, simply allow us to turn off automatically assigning workers when we create a job opening somewhere. Either way, easier access to this panel would make a lot of problems in this game go away!
Ideally, we should also be able to manually assign people to jobs without a building (and portraits of the workers in them), such as farmers.