i may have been unclear in what i meant by 'movie realism', what i was referring to was that casual level of minor-grade stunting that characters tend to perform, seamlessly rolling over a table in the middle of a dash rather than needing to navigate around it the long way, scrambling up a wall to higher ground, and yes, swinging off the hanging light fixture.
make these kinds of actions accessible and have a purpose, but not to the extent of physics defying OP-ness.
We've done stunting at my home games, but I find it tricky to balance.
The original idea was that players would propose an addition to their move or attack that would involve a skill check, and if they succeeded they'd get advantage on the next attack roll, if they failed they'd get disadvantage.
At low levels this worked out pretty well, since if you pegged the difficulty at 15, there'd usually be something like a 50% chance of players succeeding, which made it an interesting gamble.
However, as the party has reacher higher levels, a 15 difficulty check is pretty achievable. Basically guaranteed in a lot of circumstance. So then it either amounts to just letting them have advantage automatically, which takes away the excitement, or jacking up the DC, which means it's basically a treadmill.
I've sort of resolved the latter by making the DC conditional on circumstances and the nature of the opponent. So if they're fighting a big dumb brute, then yeah, it's fairly easy to earn advantage through some creative play. DC 15. But that BBEG has been around awhile, and they're not so easy to confuse with a few backflips. DC 25.
As far as rule of cool being a problem, like everything, it depends on context. The way I normally see it used is DMs trying to award creative play that goes beyond the basic moves covered by the rule set. Which has
always been a thing in D&D. It's only a problem if it the DM is too generous, because that lowers the stakes and makes the game less exciting.
IMO, you're not doing any favours by making things too easy on players - it's only cool if they've earned the advantage.
An anecdote: last fall, I ran a school campaign through the re-released
Lost Mine of Phandelver, up to level 5. When they were fighting the dragon in the ruined tower, the fighter had climbed the stairs to attack the dragon from above. It was a tough fight, and once it got low on health, the dragon tried to fly off. The player asked if his fighter could leap onto its back instead of making an opportunity attack (more or less a grapple, though grappling himself to the dragon).
So, this is an obviously awesome, cinematic moment, and I asked for a DC 15 athletics check. He made it! So now he's on the back of a fleeing dragon while the others in the party are frantically launching missile attacks and spells on a quickly escaping foe. Next round, I told him it was a DC15 to just hold on or a DC 20 to hold on AND attack. He chose the latter, and rolled a natural 20, so I gave him advantage on his attack. The kids are going crazy with excitement. He then attacks and takes a chunk out of the dragon but it still has a few HP left, and unless something happens it'll be up out of range with him on top of it and probably doomed. Which is when the artificer, the last player to go, manages to take it down at long range with a very lucky shot. The dragon crashes, and the fall will probably kill the fighter unless he makes an acrobatic check of 15 (not his forte) for half damage. He succeeds! What a moment!
That's when rule of cool is awesome - it creates those indelible moments. But they're only indelible if they could have gone either way. Because there were real stakes, each of his rolls was riveting.
So rule of cool is not the problem. Being too generous in how you arbitrate those moments can be, but that has ever been the case.
