The Usual Suspects (1995)

This face recognition scene was an impromptu performance by the actors. Originally, it was meant to be a serious scene, but the actors kept joking and teasing each other. In the end, the producers decided to keep everything as is, and it unexpectedly became a key moment in the film.
True Lies (1994)

During a striptease scene, actress Jamie Lee Curtis accidentally fell. In the background, you can see Arnold Schwarzenegger wanting to move to help her up. But the actress remained composed and continued her dance.
The filmmakers decided to keep this scene, as it suited the image of the slightly clumsy housewife that Curtis was portraying.
Pretty Woman (1990)

In the scene where Edward gives Vivian a necklace, actor Richard Gere decided to play a joke. He closed the box just as Julia reached out. As a result, this scene added a genuine laugh from actress Julia Roberts, and fortunately, it was not cut from the film.
The Lord of the Rings: The Fellowship of the Ring (2001)

In this scene, Aragorn slices a flying knife coming towards him, which looks really cool in the film. In reality, the actor opposite him threw a slipping knife, and the knife was actually heading towards actor Viggo Mortensen. But he managed to beat it with his reflexes, proving that previous sword-fighting lessons were not in vain.
From Dusk Till Dawn (1996)

In response to the vampire’s line: “Welcome to slavery,” actor Clooney improvised and said, “No, thanks. I already have a wife.”
A humorous and interesting joke, and the producers decided not to cut it from the film, allowing it to continue delighting audiences.
Spider-Man

The moment Spider-Man learns how to use his web in the scene above was entirely improvised. Actor Tobey Maguire decided that it would be a good idea to add a moment where Peter learns to become a superhero. The detail became so interesting that this scene remained in the final cut of the film.
Thor: The Dark World (2013)

A humorous improvisation from actor Chris Hemsworth, who plays Thor, when he enters his apartment and instinctively hangs his hammer on a rack. The detail fits the character’s nature so well that this scene was kept in the movie.
The Godfather

According to the script, Don Corleone had no cat. This cat simply wandered onto the set and Marlon Brando held it in his arms. As a result, a surprisingly contrasting scene emerged, with Don Corleone gently stroking the cat while discussing how to handle his adversaries.
Iron Man

Throughout the filming of The Avengers and Iron Man, actor Robert Downey Jr. often ate on set. He would frequently hide his food from the crew and eat in between takes, even while filming. Eventually, the producers gave up trying to convince him and incorporated this habit into the main character. It became another interesting trait that added to Iron Man’s charm.
Mission: Impossible 2 (2000)

An actor who was Tom Cruise’s co-star in this scene nearly swung a knife into Tom’s eye. However, luckily, he was saved by the safety cable on the knife’s handle.
Rocky (1976)

When this morning running scene was filmed, Sylvester Stallone had to run through a real market because the film’s budget was not large enough to hire people for crowd scenes. During one of these runs, a vendor threw him an orange. Stallone was unfazed, caught the orange, and thanked the stranger. This scene was kept in the film.
Harry Potter and the Chamber of Secrets (2002)

In the scene where Ron and Harry transform into Crabbe and Goyle with the help of a magic potion, they encounter Malfoy in the hallway. The following dialogue occurs between Malfoy and Harry, who forgot to take off his glasses:
– Why are you wearing glasses?
– I’m reading a book.
– Reading a book? I didn’t know you could read.
The last line was completely improvised because actor Tom Felton simply forgot his lines, but the young man was not flustered. The improvised line suited Malfoy’s character so well that it was left in the film.
Titanic (1997)

A famous fact is that DiCaprio loves to improvise and add something unique to his characters. And the famous line: “I’m the king of the world!” in Titanic was also such an improvisation.
Midnight Cowboy (1969)

The scene where the main characters almost got hit by a taxi was not in the script. Actor Dustin Hoffman was not in character (however, he was startled and dropped his cigarette), but he did not forget the personality of his character and told the driver everything he was thinking. As a result, the famous line: “Hey, can’t you see I’m walking here?!” stayed in the film.
Source: Brightside