dynamically fill a field with the value of a field in another node referenced with an entity reference field
You don't state much about your setup, so I am gonna presume you are using drupal 7. In that case I highly recommend using entityreferences. It replaces the former nodereference and userreference modules IMO. You can reference any entity with it. Also your custom CTs (Client, Contact, Job.)

When you create a reference field you have to option to use a 'simple' selection which indeed gives you ALL content of a certain type. But you can also choose to use a custom made view for the selection. 

Using a contextual filter (which you can pass along in the entityreference field settings) you can filter the view down to exactly what you want.
It's brilliant, try it!