There is a line we have to walk between 'not prepared' and 'over prepared'. In actuality, it is very difficult to be 'over prepared', but people commonly use more preparation as an excuse to delay the actual event. Over preparation is a form of procrastination!
But how much preparation is enough? This depends on the situation. Usually it is preferable to learn just the minimum necessary to perform the skill marginally well; then it is time to get into real-world application.

As an example, in my karate class, whenever we introduce a new student to sparring, they just have a few techniques. A roundhouse kick, a sidekick, and some variation on jab-cross, and they're ready to go. This is just a few out of thousands of techniques that could be used in sparring. This doesn't even bring into account variations of timing, feints, or more complicated footwork. All that can be learned later, though. It won't even make sense to a beginner if they don't have a context in which to understand it. The context they need is a few 'live' sparring matches (light contact to start, of course).

We have to have a real-world context to apply our reason. If we don't, it is just imagination. A spectator will never understand what it is to be a player (but they usually think they do).

Another example is public speaking. Make sure the speaker knows his/her material; this is the only consideration that should be given a significant amount of time before actual speaking engagements. Once they are familiar with the material, give the speaker a few tips (maybe projection of voice, friendliness in demeanor, and a quick relaxation technique to calm down), and then get them in front of an audience. Will unforeseen challenges arise? Of course! But it is much better (and less time consuming) to deal with the challenges that do arise than to pretend you can anticipate every challenge that 'might' arise.

If we have to choose one extreme or the other, I generally prefer to take action. I have been so caught up in analysis and preparation in the past that on occasion I never actually got around to performing the skill. Some preparation is good, but it can easily be taken to useless extremes.

Once you have taken action, though, you might decide to 'adjust your formula'. To use the karate example again, once you have done some 'live' sparring, you might choose to spend quite a bit of time perfecting a particular technique (maybe your sidekick) in solo practice or with a partner before you use it in a 'live' situation again. If you decide to do this, be sure to base this decision on your real-world experience.

As I have said before, the most important thing to do is take action. Once you have taken action, a world of possibilities opens up for you. Endless permutations of skill development will present themselves once you practice your skill in a 'live' environment. This often seems to be the case before taking action as well. The only difference is the possibilities that arise after taking action are based on real world experience... not your imagination!