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On-demand can be used to describe an action performed or service provided only when known to be needed. Contrast with something that is done provisionally, which would be done just in case it is needed.

Consider the on-demand printing of books, or the on-demand provisioning of computing resources, both of which are specifically intended to delay some (potentially expensive) action until there is no doubt it is needed.

Here are three related Extreme Programming practices. They avoid doing unnecessary work now, so that you can have a useful thing both now and later:

• "You ain't gonna need it", or
  
  "You aren't gonna need it."


• If you do need it,
  Do the simplest thing that could possibly work.


• The three rules of optimization:
  
  
  1. Don't.
  
  
  2. Not yet.
  
  
  3. Use a profiler.
  
  

"Yagni" is an acronym for the first concept, which means:

"Always implement things when you actually need them,

never when you just foresee that you need them."

Even if you're totally, totally, totally sure that you'll need a feature later on, don't implement it now. Usually, it'll turn out either

a) you don't need it after all, or

b) what you actually need is quite different from what you foresaw needing earlier.

This doesn't mean you should avoid building flexibility into your code. It means you shouldn't over-engineer something based on what you think you might need later on.
This also follows the KISS principle:

Keep it simple, stupid!

The page goes on to put this practice in the context of Extreme Programming's other practices. It quotes Kent Beck as writing,

"You aren't gonna need it" is not the same as forgetting experience, acting stupid, or abandoning morals. It is a challenge to developers to abandon their fears of a far-flung future that may never happen and do an absolutely stellar job of solving today's problems today.

Perhaps the terms you want are pro-active and reactive?

I remember a TV ad campaign from many years ago where the issue of preventative maintenance was addressed. A mechanic in a garage is addressing the audience and mentions a customer who had a loose bolt holding the muffler on his car who didn't do anything about it; eventually, the muffler fell off. The customer paid a lot more to replace his muffler than he would have had to pay to get the bolt tightened. The mechanic's final line was "You can pay me now or you can pay me later...."

That's not QUITE the same thing as the original question was asking - a loose bolt will inevitably lead to a muffler falling off whereas the work the pro-active person does in the original question will NOT necessarily prevent ANY problem - but it's in the same territory.

Hmm. Maybe instead of pro-active, a better word might be anticipatory. The anticipatory person wants to be ready for any contingency, even if the odds of that contingency happening is very slight. The reactive person doesn't want to waste a lot of time on what ifs so they just react when and if a problem does occur.

I'm not aware of a precise, unambiguous term for it. As the other answers show, there are terms for certain contexts, but nothing general.

Personally, I would go with pragmatic vs prepared.

A person that is pragmatic about his or her work will do what is necessary and avoid doing things that might not be needed, while someone who is prepared about his or her work will do things in advance on the chance that they might be needed in the future.

In programming you would call this lazy. For example, lazy evaluation of an expression means you will only evaluate the expression if and when it's needed.

The word lazy has a negative connotation, so it might not be suitable in other cases.

Both parties have planned their time in advance, the only difference is that the first party has planned everything whether it is important and necessary to them or not, while the second party has planned only the substantial and needed things. This all leads me to think of the verb "prioritize" which means:

To decide which of a group of things are the most important so that you can deal with them first.

Cambridge Dictionary

It is an expression that is very common to use when dealing with time. So, in order to use your time effectively as much as possible, you should prioritize your work.

• Prioritizing has never been a last minute thing to do; thus it has implicitly the indication and meaning of planning your time beforehand productively.

Some good answers so far, but I'll also submit:

At need: only doing something when it's necessary to do so. (This is a little bit uncommon in everyday, U.S. English, and will probably read as formal).

We have our set of reports that we write each month, but we also have some that are produced at need.

There are lots of variations on this which you might encounter, like as necessary. Jasper pointed out in comments a couple more, including when needed and as needed, the latter of which especially is very commonly heard.

Doing something at the last minute is another common expression.

Packing for a trip, "I'm going to leave the snacks in the fridge until the last minute so they don't spoil."

A teacher might warn a student "This project will require a lot of time; don't leave it to the last minute."

When used as part of the larger expression doing everything at the last minute, it implies the subject is lazy, unmotivated, overworked or otherwise struggling to meet objectives.

Depending on what you're doing, this idiom might be useful:

We'll cross that bridge when we come to it.

an expression that means you will not worry about a possible future problem but will deal with it if it happens - Cambridge Dictionary.

Essentially, you don't put in time or effort into (or worry about) a possibility, until you're actually at a stage where the possibility is nearly a certainty.

One informal expression which I think comes pretty close is doing things "on the hoof". However, it seems to be chiefly a British expression. Here's a definition from Collins via thefreedictionary.

1. If you do something on the hoof, you do it as a quick reaction to something that has happened, rather than planning it carefully. They claimed that policy was being made on the hoof. In that situation, you have to make decisions on the hoof.
   

I believe this is a phenomenon c
known as "planning."

I am not familiar with the nuances of the event, but I understand those who observe experience a lower degree of stress than Master Procrastinators.