If the client comes in depressed because she "had a tough week", you might be able to cheer her up by telling her a funny joke. Maybe she would even stay in a good mood for the rest of the session. And if there really wasn't much to her depression in the first place, that might be all it takes. But most likely a deeper handling would be more appropriate.
To handle an issue more deeply you would want to get some idea of what it is about, why it is there, how it is being maintained, what experiences did it entail, what is there to learn from it, and so forth.
We could say that we are after the reason for a certain issue. That we are getting a reason is however nothing more than appearance. There can be reasons on many levels. There is no one reason why a client has a certain aberration in her life. "Just because" is just as good a reason as anything else. But, it can be noticed that the deeper we dig the better and more complete the reasons seem to be.
Some of our techniques are very light, some are more heavy duty. That doesn't mean that the more industrial strength tools are better, it all depends on what is needed. Sometimes a band aid is most appropriate, sometimes major surgery is.
Dialoguing is a fairly light action. A simple positive suggestion is lighter. Re-experiencing is deeper.
Don't think that just because you do re-experiencing and the core incident gets cleared that you have found the real "reason" for the issue you are handling. It might be a more convincing reason than what a simple dialogue turns up, but that doesn't make it the ultimate truth.
Actually any kind of "reason" you find in a process simply acts to convince the client that she should change her mind right now. A deeper process just provides a more convincing set of reasons for the problem and the more compelling a reason to change. It all still comes down to the client's considerations.
What kind of technique you choose depends on what you notice about the client's situation, but also on time constraints. You should always make the client feel good at the end of the session. If she brings up a major item five minutes before you planned to end the session, then you might prefer to give it a band aid or simply to get the client to think about something else. Then when we have more time we can get back and actually work on it another time.
Many positive techniques, like visualization or grounding, ignore what the client's precise state is, and simply make her feel good. That doesn't really clear any issue to any depth, but it does accomplish the intention of processing. We could say that a grounding process is a superficial process, but that isn't quite fair. It is another type of process, and both are needed.
If a client is only treated to deep processes that find important sounding reasons for everything, then she would tend to go too much into agreement with that overall idea. That is not very healthy, it would give sort of a negative orientation of processing.
It is advisable to apply processes of varying depths. Don't leave the client or yourself with the idea that there is only one correct degree of clearing things.