▲In the classic war film, the director took advantage of actor R. Lee Ermey's military background in order to flesh out the character of the potty-mouthed drill sergeant. The result was a lot of improvisation based on Ermey's real-life experiences.

▲Hannibal Lecter is known for the famous line, "Once a census taker tried to test me. I ate his liver with some fava beans and a nice Chianti." But the final touch in that scene, the strange noise he made afterwards, was a spur-of-the-moment choice.

▲There was a lot of improvisation in Stanley Kubrick's classic film, but one of the standout moments was when Peter Sellers rises from his wheelchair, saying "Mein Führer! I can walk!"

▲When the situation got tough in the film, Bill Paxton really felt the pressure, going off script when he said, "Game over, man. Game over!"

▲The famous scene in which Mr. Blonde cuts off Marvin's ear was not described in the script, so Michael Madsen was free to improvise, putting on a show and joking around with a dance.

▲One of the most memorable moments from the film is when Robin Williams discusses his wife's unconscious flatulence, but the improvisation worked splendidly and the two stars' fit of laughter was genuine.

▲Paul Rudd and Seth Rogen also improvised every "You know how I know you're gay" line, which worked so well that they repeated it in the movie 'Knocked Up.'

▲The moment when Jim Carrey asks Jeff Daniels if he wants to hear the most annoying sound in the world, and then lets out a weird scream, was a scene that was not written in the script.

▲The classic romance, directed by Michael Curtiz, is known for its immortal line: "Here's looking at you, kid." Little do most people know, Humphrey Bogart improvised it!

▲Ryan Gosling and Emma Stone's intoxicating love scene, the pillow talk following the 'Dirty Dancing' bit, was almost all improvised, and the director said they just poured a couple drinks and filmed them for a really long time.

▲Most people don't know that in this film Joe Pesci reenacted a real moment in his life from years before, when he made the error of telling a gangster he was funny.

▲In the lineup scene at the police station, the actors were simply supposed to step up and repeat a line, one by one, straight-faced. Apparently Benicio Del Toro kept farting and no one could get through the scene without laughing.

▲Gene Wilder added a surprise, limp-to-somersault entrance into the film, which he felt was necessary to the character, since no one would know if he was lying or telling the truth from that point onward.

▲The entire chest-waxing scene was completely authentic, as it was Steve Carell's first time being waxed, and every hilariously creative swear word was a genuine response to the pain.

▲The classic line "You're gonna need a bigger boat," often remembered incorrectly as "We're gonna..." was entirely improvised by Roy Scheider, and has now gone down in history as one of the most memorable lines from the movie.

▲Ralph Fiennes changed Voldemort's final speech with every rehearsal, and so the reaction from the rest of the cast was different and authentic for every take.

▲When Aragorn kicks a helmet out of frustration, he lets out an unscripted roar that was actually the result of actor Viggo Mortensen breaking his toe.

▲When Harry and Ron transform into Crabbe and Goyle, Goyle is wearing glasses and Malfoy, AKA Tom Felton, asks why. Goyle says he was reading, and Felton, in place of a forgotten line, delivered one of Malfoy's most iconic lines: "I didn’t know you could read."

▲Donald Faison heard the phrase "Keeping it real" from a neighbor and decided to throw it into the film to make his character seem more like he was actually in high school.

▲When Tom Hanks exits the bookstore while carrying balloons and a goldfish, the balloons accidentally get caught in the door and he says, "Good thing it wasn't the fish!" That was pure Hanks.

▲In the scene where Han Solo is taken away to be frozen in the graphite machine, the character was supposed to reply "Me too," in response to Carrie Fisher's "I love you," but Harrison Ford changed it to "I know," to be more fitting for his character.

▲The infamous scene where Maya Rudolph runs across the street in a wedding dress and proceeds to poop on the street was somehow born from the actress's imagination!

▲Leonardo DiCaprio accidentally cut his hand open while filming a scene but decided to keep acting anyway, even smearing his real blood all over Kerry Washington's face. When the scene was over, the cast and crew gave him a standing ovation.

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The script included a lengthy sword fight between Indiana Jones and one of the bad guys, but Ford was suffering from dysentery and could only film in 10 minute increments, so he changed the scene at the last minute and just shot the goon, cutting out what would have taken two to three days to film.

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The original screenplay set Arnold Schwarzenegger's character to be blown up at the end, but instead the actor said, "I need a vacation" and the rest is history.

▲Timothée Chalamet improvised that final moment of the film where his character looks emotionally into the camera's lens. He said it was an homage to the film 'Boyhood' where the main character stole a two-second look directly at the camera.

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The final iconic and cult-favorite tears-in-rain monologue—"All those moments will be lost in time, like tears in rain"—was largely improvised by actor Rutger Hauer!

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Much of the spontaneous clowning in the lobster-cooking scene was truly spontaneous, as was the scene where Alvy sneezes away a fortune's worth of cocaine at a party with Annie.

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After recording the same scene numerous times, the director gave Malcolm McDowell the freedom to do what he liked, so he danced and sang, all improvised.

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While filming the movie in the streets of New York, a taxi reportedly jumped a light and almost hit Dustin Hoffman and Jon Voight, which is where the spontaneous and memorable line, "Hey, I'm walking here!" came from.

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Few people know the origins of the scene-stealing cat in the film which wasn't in the script, and many say that Francis Ford Coppola put the creature on Marlon Brando's lap just before they started filming.

▲In the scene where the Joker leaves the hospital, the bomb is supposed to go off right when he presses the detonator. Apparently it didn't work, but Ledger made one of the film's most comic points by continuing to act and fidgeting with the detonator until the explosion went off.

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Robert De Niro was only given the instruction to speak to himself in the mirror, but his improvisation became one of Hollywood's most iconic moments.

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In an interview, director Garry Marshall revealed that Richard Gere's snap of the necklace box wasn't planned, which made Julia Roberts's surprise and laughter completely genuine.

▲In the brief meeting between Tom Hanks and Matt Damon, Ryan (Damon) delivers a kind of monologue with stories of his brothers and a girl back in the US, all of which Damon made up off the top of his head.

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Margot Robbie took her chance while filming a scene with Brad Pitt for Damien Chazelle's 'Babylon' by suggesting a kiss that "wasn't in the script." The actress told E! News that she improvised the scene with the help of the director. "I thought, 'When else am I gonna get the chance to kiss Brad Pitt? I'm just gonna go for it.'"

Robbie stars as budding actress Nellie LaRoy and Pitt plays actor Jack Conrad, in the film, set in 1920s Hollywood. "I said, 'Damien, I think Nellie would just go up and kiss Jack.' And Damien was like, 'Well, she could—wait, hold on. You just wanna kiss Brad Pitt,' " Robbie recalled. "And I was like, 'Oh, so sue me. This opportunity might never come up again.' And he was like, 'It does work for the character,' and I was like, 'I think so.'" Even more conveniently, after one take the director was sold on the idea. "He was like, 'No, do it again. That really works.' I was like, 'Oh, great,' " said Robbie.

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The terrifying scene in which Jack Nicholson breaks down a door with an ax is known for the improvised line, "Heeere's Johnny!" which Nicholson admits to borrowing from 'The Tonight Show Starring Johnny Carson.'

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A good script is supposed to have incredible moments and carefully crafted lines, but sometimes it takes an actor deep in character to fill the space between the lines with realistic, memorable moments that were born solely from the momentum of the scene. Sure, improvisation doesn't always work out, and many unscripted scenes have been cut from the final product, but when it does work, it has proven to be legendary.

Take a look at these famous scenes that were improvised by actors.