Brad Pitt's Acid Trip Wasn't In The Once Upon A Time In Hollywood Script
I am someone often annoyed by scenes in movies where a character is high. The scenes often feel like the filmmaker thinks they are the first person to ever put this on film, yet almost everyone does it the same way. There will be direct point-of-view shots, a warping effect on the image, and the sound will slow down. This is especially true in a lot of comedies, and no one ever goes past the thought, "Isn't it funny they are high?" No, it is not inherently humorous. You need to put your own spin on it.
Take last year's "The Worst Person in the World," which features a scene where a group of characters do mushrooms. Joachim Trier designs plenty of images rarely found in scenes like this, such as Renate Reinsve's Julie being so out of it that she removes her tampon, throws it across the room, and uses her menstrual blood as some kind of warpaint on her face. Doing something that extreme catches your attention and really makes you laugh. Trier found a new way to craft a tripping scene that both is quite funny but also weaves in plenty of thematic concerns of the film.
Another way to do make an interesting tripping scene is to not make it about the tripping. This is what Quentin Tarantino does in the climax of "Once Upon a Time ... in Hollywood." Brad Pitt's Cliff Booth has just smoked an acid-dipped cigarette when three members of Charles Manson's gang break in to murder Leonardo DiCaprio's Rick Dalton. In his high state, all Cliff can do is laugh. It's a vital comedic layer to an incredibly intense scene, and somehow, it originally was not even in the script.
Amplifying the absurdity
The scene where the Manson cronies burst into the home was originally not that different than what's in the final film. Brad Pitt's Cliff was still going to find the whole situation rather funny, but it came more from his nature of nothing really phasing him and finding the idea of people wanting to prove themselves a little silly. We see this earlier in the film with how he responds to Bruce Lee's (Mike Moh) ego.
As they were coming closer to shooting the scene, Quentin Tarantino came up with the added bonus of the acid-dipped cigarette and Cliff being high during the whole sequence. Talking with the New York Times, Pitt recalled when Tarantino came to him about the new layer to the scene:
"He came up with that a couple of weeks before we started shooting. We already had the script, and then he said, 'You're going to be on acid in that scene.' I said, 'Great!' It gives you so much room. The clichés of acid trails; it's always funny. Everyone gets it."
For as reverent as Tarantino is to what he puts on his own script pages, it is nice to see he still has the ability to know when to play with something after it's been on the page. A lot of writer/directors get so locked in about their material that simply changing something somehow discredits their own purported genius. And I'm sure there's plenty of that with Tarantino too, but in this instance, he knew how to shake something up.
As for Pitt's actual experience with LSD to draw upon, he says in the interview, "I'm microdosing right now." He was laughing when he said that, but Pitt seems chill enough to a point where he certainly could have been.