While working in a commercial gym, a lot of my colleagues went to great lengths to plan out their PT sessions ahead of time. Meanwhile I spent my time wondering why they bothered. I know that sounds like a really slack thing to say, but it's true.
A lot of PT's spend a lot of time perparing each individual session. When you're in a private environment, I can see the merit to this. When you're in a gym with 3,000 members, training your clients in peak hour, I can't.
My reasoning is simple. The more variables you can control, the more specifically you can plan, and the more potential you have to stick to the plan. The less variables you can control, the less specifically you can plan, and the less potential you have to stick to the plan.
Here's a specific plan for a workout:
Back Squats 5x5
Rack Pulls 5x5
Bench Press 3x10
Seated Rows 3x10
Now let's get to the gym in time for the session to start. The squat rack (or racks, if it's my kind of gym) is taken, and it'll be a lot of hassle to work in. You can spend time negotiating with the current occupant of the rack and changing weights, but you're on the clock. Oh look, we've got a bunch of dumbbells available, should we perhaps do 3x10 goblet squats instead?
Goblet squats are finished, now it's time for rack pulls. But oh look, the rack is still in use, and trying to set up rack pulls in an occupied squat rack that's being used for its intended purpose will cause even more hassle and more time-wasting. But there is still a free barbell and plates. Now we can do deadlifts from the floor.
After deadlifts, it's time to bench. But OH NOEZ, a horde of 16yo boys have clung to all 3 bench stations and have created an impenetrable barrier around the benches. But this is interesting, the pull up/dip stand is free. I wonder what we should do? Gosh oh golly, I just had a great idea, let's do alternating sets of dips and pull ups.
And there you have, your session completed in a way that completely didn't adhere to the session planned. MADNESS, I say! But, there's a method to this madness, and it's a method that should be quite clear. One kind of squat wasn't available, so we did another. One kind of hinge wasn't available, so we did another. One kind of push wasn't available, so we did another, and we worked in an unplanned pull while we were at it.
It should go without saying, but the more specific your client's goals (or your own goals), the more specific the programming has to be. I'm competing in my first powerlifting meet in about 10 weeks, and deviations from the key parts of my program are very likely deviations from how well I can compete when the time comes. Fortunately for PT's, most clients' goals are a little less specific and a little more general, which means that the PT has the freedom to step away from the plan to some degree. I figured this out long before I had my first paying client. You just can't control all the variables, or even most of them, in fact you can barely control any of them. Consequently, I spent time behind the scenes figuring out what exercises can be swapped in for something else and elicit a similar effect, rather than figuring out how each session should go, knowing that it probably won't go according to plan anyway.
So I took to introducing a little madness, but with a method to it.
Method is good. It gives programming something reliable. But if method is all you've got, you're going to give yourself and your clients a lot of undue stress (or distress) when the inevitable happens and the method can't be practically reconciled with the reality of the environment.
Madness is good. It allows flexibility and variance. But if madness is all you've got, there's very little you can be sure of. If your workouts are completely random, then the odds of them being appropriate to your client's goals, needs, wants and capabilities is drastically reduced.
Method on its own works perfectly on paper and poorly in practice, because the conditions required to make it work are seldom there. Madness on its own works poorly both on paper and practice because it seldom hits the mark, however on the off chance the it does hit the mark, it does start helping you get somewhere. But incorporating the two together can be the next best thing to a magic pill in trainer-land.