Lots of making prep seem bad around here, and I think that is a bit odd. Something to note when we are discussing the use of prep. A growing body of research shows that working memory, the memory used in improvisation, has limits. This is often referred to "working memory capacity" or WMC. This was given as the "magical number seven" in the 1950s.
There is the old myth that phone numbers are seven digits long, in the US, because of this limit. That isn't true, but most research puts this limit around seven numbers for an average person's working memory capacity. Onaverage a person can repeat back roughly 7 numbers they were given moments prior in an experiement. After that you see errors, with increasing frequency. With extensive training, you can short cut this, but there is always a limit.
So why does this matter? Well it means that there is a limit to your improv skill. Reseach shows us that as you approach this limit, under high cognitive load, your creativity declines. This is because the additional mental effort required for a task can overwhelm working memory, making it harder to generate or process novel ideas. Often, for us DMs, this is when a chest has nothing of note in it, or when we resort to a cliche or other predictable outcome. You might also forget names, voices, or other details. Watching for these in a DM can be amusing.
It might be better to engage in some prep instead of forgoing it completely out of fear of a negative outcome. Because by relying on only working memory through improv, you are essentially bottlenecking your creativity from the start. Not engaging in prep is simply a handicap. It would be better to prep in a way that avoids any agency issues, so as to still reap the benefits of using more of your brain.
Sources:
Anecdotes from creative eminences suggest that executive control plays an important role in creativity, but scientific evidence is sparse. Invoking the Dual Pathway to Creativity Model, the authors hypothesize that working memory capacity (WMC) relates to creative performance because it enables...
pubmed.ncbi.nlm.nih.gov
The relationship between the ability to maintain task goals and working memory capacity (WMC) is firmly established, but evidence for WMC-related differences in conflict processing is mixed. We investigated whether WMC (measured using two complex-span tasks) mediates differences in adjustments...
pubmed.ncbi.nlm.nih.gov