I'd say the psychology and politics of testing are different from other things, really quite strange.
It most white collar work, there is strong pressure to tell people what they want to hear. Well, it makes me very happy to get a clean bill of health from a tester, but if I was expecting that to happen, I wouldn't need a tester.
Testing takes a kind of special discipline that the average office worker doesn't have. Whenever I've been an organization that has tried to press ordinary people into a tester role, it's always been highly ineffective.
That said, there isn't one formula to make a good tester. Some testers I know are very systematic, they belong to quality control associations, others are expert in project management, requirements, etc. On the other hand, one of the best testers I worked with had bombed out of every other job we gave him because he was always able to screw up anything we gave him. As a tester, he was great, because if he couldn't break it, nobody else could.
All of these things, I think, point to a need to a longer-term relationship with a tester than you're going to get in a freelance situation.