The Usual Suspects
While there are directors who don’t mind ad-libbed lines, others completely prefer to stick to the screenplay. Brian Singer generally belonged to the latter category, though he did relent in one scene in The Usual Suspects. In particular, we are talking about the lineup scene in the heist movie.

The Usual Suspects
Apparently, it was meant to be played straight but the others couldn’t help laughing since Benicio Del Toro had some gas problems. The scene was eventually played for jokes since everyone had a hard time keeping a straight face with all the farts!
Goodfellas
When we talk about gangster films, it is pretty impossible not to mention the fan favorite Goodfellas. It is right up there with The Godfather trilogy, after all. None of this would be possible without the great cast composed of Robert De Niro, Jose Pesci, Lorraine Bracco, and Ray Liotta.

Goodfellas
However, Pesci deserves the award for the best improvised line. As Tommy, he came up with the “Do I amuse you?” line all by himself! Major kudos to him for his brilliance.