When studios have tens of millions of dollars riding on a film, and a director has his image and reputation on the line, actors are often hammered to stick to the script exactly as written. Many great directors, like Alfred Hitchcock, felt that actors should stick to the business of acting and leave the script and story to others.
Yet when actors are in their character and get a chance to add their own flavor, the result is sometimes magic.
Many of the lines in this video are iconic pieces of cinematic history. Thankfully directors recognized the gifts they were given and kept them in.