I think about training all the time. I plan different strategies. You would think I have it down by now but it always evolves. You have to adapt to changing work situations, family obligations, age, and yes, exhaustion.
I train according to my planned out training cycle in a journal like a script I try to follow. I keep a training diary of weight used, reps and time to complete a workout. It does seem that the majority of successful bodybuilders just wing it when they go to the gym. If 200lbs feel right they go with it. If today is 180lbs feels right then 180lbs it is. Guys that follow a script like myself will say last work out I used 200lbs so this workout it's either 205lbs or one more rep. It becomes really stressful. I also envy guys that change exercises on a whim. I change exercises but it's planned. I wish I could be less anal about working out and just do it intuitively.