One big factor is DM style for resolving player actions.
If they say they jump onto a table and attack from the high ground do you in 5e give them advantage for being action movie swashbucklery or do you make them roll acrobatics to successfully jump on the table with them falling prone if they fail?
Either sets a tone, but the former rewards action and the latter penalizes doing stuff.
This is the way.
@bloodtide
The preoccupation with "realism" and "grittiness" and "hard encounters" etc., etc. teaches players three things:
1. If it doesn't work precisely the way the DM thinks the real world works, it will either always fail, or be nearly guaranteed to fail.
2. Failure is
extremely bad and will almost always result in severe, often fatal, consequences.
3. Magic, because it
can't be compared to real life, is MUCH easier to use, and much safer.
Between those three, it teaches all but the most aggressively risk-seeking players to never, ever try. Trying is for people who want to die. Meekly avoiding is
the successful survival strategy. It will never be particularly enjoyable or engaging. It will lead to empty, boring play experiences. But it is what these games reward doing (because survival is such a rare and precious prize in these games), and thus most players will follow the rewards.
If you want players who embrace creativity and consider the things on their sheets, you have to:
1. Show them that failure doesn't have to be horrible. That failure can even be
fun, in the right contexts, leading to more game and better game, not less game and worse game.
2. Prove to them that creativity actually works. That it's worth the risk, and not "suicide with more steps." This means making the rewards good,
and making the punishments tolerable.
3. Give actual examples to follow.
Tell them that they have tools they can use, and
show them what they can achieve. With time, they'll learn to use that thinking themselves.
4. If possible, work with an experienced player, who can act as a model for the others. Support that player's efforts to
show what a clever character can achieve even without magic.
Ultimately, though, you reap what you sow. If you sow a world where everything is deadly, you will reap players will choose to do the things that keep their characters alive, or rather, the boring inaction that keeps their characters alive. If you sow a world where creativity often fails or pays paltry benefit, you will reap players who think creativity is pointless. If you sow a world where mundane methods have a high bar to clear just to
attempt them, let alone to
succeed at them, you will reap players who look at mundane tools and think, "None of this has any use here."
Give good, desirable rewards for the deeds you want to see from your players. Avoid making desirable actions unlikely to succeed, unless you have an
extremely good reason, which should be rare. Let players fail without having their characters pay horrible (perhaps ultimate) prices for such failure.
You will find that players respond. If you have already taught them that failure is very very bad and death lurks behind every dice roll etc. etc., then it will take time (possibly a very long time) for their behavior to change. But it
can change,
if and only if you show that it's worthwhile to take the risk.
Until then? Your players won't trust you not to screw over their characters when they try something creative.
Edit: Unless, of course, your goal is to
make players who are eager risk-seekers. I'll save you a lot of time if that's your goal. It won't work. You will not make the risk-averse into risk-seekers. All you will do is frustrate both yourself and your players. There is nothing you can do, within the context of your "Hard Fun," that will make every player eager to take often-lethal in-character risks. You will merely have to settle for getting only a small percentage (perhaps a quarter) of all prospective players, because those are the people who were already, inherently risk-tolerant or even risk-seeking.