Why do some people play out every possible scenario in their head when contemplating how to react while others give it little to no thought?

Fear of failure/doing the wrong thing.

I do it because I hate feeling “stupid.” I try to assume all possible outcomes, and have at least some preparation for each outcome. Of course, I have my favorite outcomes, and my least favorite outcomes, but I try to asses which will achieve the results I want with the best effort and effect.
I rarely ever just “speak out,” when making a decisions, because I am afraid someone will call me a moron or call me stupid, if I don’t contemplate every possible, or at least every probable outcome.
Some people can do this, some people prefer this. I don’t know why, though, honestly, because I don’t think like them. I surmise that they do not have this fear of failure, as I do. Or perhaps they don’t care about consequences, they just want outcomes. I really don’t know. But I always try, if I can, to foresee outcomes.

Some people tend to over analyze situations. They generally do it because they have more neural connections and capacity than those who react with little or no thought.

Except for those times when those who seem to react with little or no thought have actually thought things through and are simply quite quick in coming to a decision. These people may actually have more neural connections.

It weeds out those who want to live forever from those who want to live in the moment.

Personality type and confidence.

Some personalities are impulsive and other methodical. Both are influenced by the level of confidence the person feels.

I’ll take impulsive over methodical any day of the week. lol
My neurotransmitters are at warp speed. lol

This is a great question. It’s one that probably crosses our mind on a regular basis. There are two factors: Personality and Experience.

As a few of the above answers state, there are different personality characteristics, or what has been labeled as Strengths by The Gallup Organization.

People who tend to be naturally Strategic or Deliberative tend to process the potential scenarios before making a move. Those that appear to ‘wing it’, often are strong with the Activator Strength, and possibly Achiever.

While all of these characteristics often end up with the same result, the processes taken can look very different. When any of these Strengths are overused, they become a weakness.

One of my top Strengths is Activator. Like @Coloma, I tend to want to accomplish a task as soon as it has my attention. In some cases, it works. It can even be very effective. In others, it is really not the right time. Maybe an important step is skipped or it isn’t the right time. People who are Strategic map out all of the right steps, but gee, sometimes the result gets dragged out. I’ve found that by working with someone who has a Strength that counter-balances mine, we end up with an effective and punctual plan/result.

The other factor is that some people can act more quickly purely based upon applying previous experiences. Let’s say that you and a friend travel to a country that neither of you have been to before. The city where you are staying has a metro system. One of you has used a similar system in another city and has no fear of getting on it and using the maps on the train to find the desired destination. The other wants to learn about it on the internet before entering a station. Thus, the confidence level comes into play, even if it may result in a mistake.

It all comes down to communication, doesn’t it? It’s just a matter of letting the other person know when a possible decision is outside of the comfort zone and agreeing on how to handle it. This works in both professional and personal scenarios.

And now I am curious…what brought on this question?

I think it is closely related to whether you are an introvert type or an extrovert type. Introverts analyse before they do, extroverts do then analyse (in general, of course). Just different strokes for different folks.

@Stinley Yes, extroverts tend to improvise as they go, introverts tend to need a solid plan.