Black-Box vs. White-Box

Black-box testing or behavioral testing is testing based upon the requirements and, just as the name implies, the system is treated as a "black box." That is, the internal workings of the system are unknown. In black-box testing the system is given a stimulus (input) and if the result (output) is what was expected, then the test passes. No consideration is given to how the process was completed.
In white-box testing, an input must still produce the correct result in order to pass, but now we're also concerned with whether or not the process worked correctly. White-box testing is also called structural testing because it's based upon the object's structure.
The creation and execution of tests is best be done by the people who understand the environment associated with that level of test.
Creating automated test scripts can often take more expertise and time than creating manual tests. Some test groups use the strategy of creating all tests manually, and then automating the ones that will be repeated many times. In some organizations, this automation may even be done by an entirely separate group. If you're working in an environment where it takes longer to write an automated script than a manual one, you should determine how much time is saved in the execution of the automated scripts. Then, you can use this estimate to predict how many times each script will have to be executed to make it worthwhile to automate. This rule of thumb will help you decide which scripts to automate. Unless there is very little cost in automating the script (perhaps using capture-replay, but don't forget the learning curve), it's almost always more efficient to execute the test manually if it's intended to be run only once.