You can prepare well by understanding the basic principles of processing, and by knowing many different techniques, and by having experience in what works and what doesn't. But once you are sitting there in session what matters is what you actually do with the client there. She is different from everybody else. She is even different from herself in the last session. You need to deal with her the way she is. Not the way she is supposed to be, or the way she used to be.
Twenty years ago people didn't change so much. It was quite reasonable to expect that somebody comes in after a week without having changed in any significant way. That is no longer likely. People live between sessions, they do things, they learn things, they run into trouble.
A client can gain even from a generalized program that is intended to match a cross-section of the population. We will probably put her on such a program, if she is interested, after we have handled her major life issues. But we can't expect that we can plan everything in detail in advance. We can't expect that we know exactly how she will behave.
It is not so much that you can't persuade people to be treated all the same. You could. It just takes a bit of indoctrination convincing the client that there is only one right way of getting better. But it isn't going to work as well.
The person is leading his own life. Whatever he is right in the middle of is usually a pointer to what he should be looking at. The biggest gain is usually connected with what is right there. If he has trouble with his wife, well, then there is probably some good stuff to find on that. If he has a lot of attention on his work, then we should probably help him with that.
Not only is it the easiest to work with what the person is in the middle of, it also produces the biggest gain. The client will be interested and communicative, i.e. involved in the session. The results will be immediately apparent in her life and will be more permanent. Whenever the client has an issue close at hand that holds her attention, take that up as the first thing.
Even when you are using a general technique, e.g. part of a module, you still work with exactly the client in front of you. You need to adjust to where the person is at. You need to be sure to handle the exact loaded areas that come up. Don't just keep giving the next direction; notice what is happening. The activation loop is that you bring something up, deal with it, and then bring something else up. Even if it is a repetitive question you need to deal with what each question brings up separately. It might be an upset, a concern, a mis-understanding, a feeling, a fixed idea. Deal with it and then go back to the process. Usually a simple acknowledgment or a little bit of dialoguing is enough, but sometimes you might have to resort to a different process, e.g. re-experiencing.
You, the facilitator, need to be present with the client. You need to have the majority of your attention on her, what she is doing and saying, how she is responding. You need to notice what is different about what she is doing, as well as noticing what is similar to other clients you have experienced or principles you know of.
You also need to be flexible enough to deal with what is happening. You need to have some choices present and you would take the best course of action that fits the way you perceive the client. You need to have a tool that approximates where the client is at. You don't have to do things perfect, but you do have to be conscious of where the client is at and what you are going to do about it.
The theories and models of processing are what you work with out of session. But once you sit down with the actual client you work with the actual person there. The only thing you can never be faulted for is dealing with the person who is there.