It's a well known an often repeated story that Han Solo's last words to Princess Leia before being frozen in carbonite at the end of The Empire Strikes Back were improvised by Harrison Ford. It turns out that story isn't entirely true. While it is true that the line "I know" wasn't in the original script written by Lawrence Kasdan, Ford didn't improv the line on set but instead talked over his suggested change with the film's director beforehand.
The original script dialogue was as follows...
Leia: ...I love you. I couldn't tell you before, but it's true
Han: ...just remember that, 'cause I'll be back.
As dialogue goes, it's not terrible. Han's response is still a little cocky and self-assured, although, perhaps a little too much for the situation. However, according to recordings of the set of The Empire Strikes Back as reported by Vulture, Harrison Ford explained to director Irvin Kershner that Han responding with "I know" better fit the moment and Han Solo as a character. The director agreed and the rest is slightly misreported history.
It's interesting how the story of the line being improvised entirely by Harrison Ford has become so widespread if it isn't actually true. Fans of movie trivia, especially around Star Wars, have always had that one available. It would appear that somewhere along the line, when it was discovered that "I know" wasn't the line in the script it was assumed that that Ford must have been the one to change it. Quite often dialogue gets changed from the page to the screen, although most of the time it's changed by the director or the screenwriter themselves. When the director knows you're going to say something other than what's on the page, it's not really improvisation.
It would appear that there were no hard feelings on the part of the screenwriter for not using his dialogue. When Lawrence Kasdan and George Lucas collaborated on the script for Star Wars: The Return of the Jedi they ended up using the dialogue again, though, in a brilliant stroke, they reversed the characters. Honestly, Leia delivers the line better.
So while it's true that you can thank Harrison Ford for one of the most memorable lines in movie history, It's not one of the greatest improvised lines in movie history. While this fact makes the story a little less exciting, it doesn't change how great the scene is. Do you have a favorite movie moment that wasn't in the original script? Let us know in the comments.