The Usual Suspects (1995)

This facial recognition scene was actually an improvised moment by the actors. It was supposed to be serious, but the cast kept joking around and teasing each other. The producers decided to keep it, and it ended up becoming a key moment in the film.
True Lies (1994)

During a striptease scene, Jamie Lee Curtis accidentally fell. You can see Arnold Schwarzenegger moving to help her up, but she quickly recovered and kept dancing. The filmmakers kept the take because it fit her character’s clumsy housewife vibe perfectly.
Pretty Woman (1990)

When Edward gives Vivian a necklace, Richard Gere played a prank by snapping the box shut just as Julia Roberts reached out. Her genuine laugh made the scene even better, so it stayed in the final cut.
The Lord of the Rings: The Fellowship of the Ring (2001)

In this scene, Aragorn slices a flying knife in midair. It looks amazing on screen, but the actor throwing the knife actually missed his mark. The blade was heading straight for Viggo Mortensen, who reflexively blocked it, proving his sword training paid off.
From Dusk Till Dawn (1996)

Responding to a vampire’s line, “Welcome to slavery,” George Clooney improvised, saying, “No thanks, I’m already married.” The joke was so funny the filmmakers kept it, and it still delights audiences today.
Spider-Man

Tobey Maguire improvised the moment where Peter Parker learns to shoot webs. He thought it would be cool to show Peter figuring out his powers, and the scene was so good it stayed in the final film.
Thor: The Dark World (2013)

Chris Hemsworth added a funny touch by casually hanging his hammer on its stand when entering his apartment. The moment fit Thor’s character so well it was kept in the movie.
The Godfather

Don Corleone wasn’t supposed to have a cat in the scene. The cat wandered onto set and Marlon Brando just held it. The unexpected addition created a striking contrast as Don Corleone gently stroked the cat while talking about dealing with enemies.
Iron Man

During filming of The Avengers and Iron Man, Robert Downey Jr. often snuck food and ate on camera. The producers gave up trying to stop him and made this quirky habit part of Tony Stark’s character, adding to Iron Man’s charm.
Mission: Impossible 2 (2000)

In this scene, Tom Cruise’s co-star nearly stabbed him in the eye with a knife. Luckily, a safety cable on the knife’s handle saved the day.
Rocky (1976)

While filming the morning run scene, Stallone had to run through a real market due to budget limits. During one run, a vendor threw him an orange. Stallone caught it smoothly and thanked the stranger. The moment made it into the movie.
Harry Potter and the Chamber of Secrets (2002)

When Ron and Harry transform into Crabbe and Goyle, they run into Malfoy in the hallway. The improvised exchange goes:
– “Why are you wearing glasses?”
– “I’m reading a book.”
– “Reading? I didn’t know you could read.”
Tom Felton forgot his lines but stayed in character perfectly, and the improvised insult fit Malfoy so well it stayed in the film.
Titanic (1997)

Leonardo DiCaprio loved improvising to add unique touches to his characters. His famous line “I’m the king of the world!” in Titanic was one of those spontaneous moments.
Midnight Cowboy (1969)

The near taxi accident scene wasn’t scripted. Dustin Hoffman wasn’t fazed but startled enough to drop his cigarette. Staying true to his character, he shouted at the driver, “Hey, can’t you see I’m walking here?!” The line became iconic and stayed in the film.
Source: Brightside