tinsnip:

“Babylon 5: A particularly interesting form of ad lib occurred in the third-season episode “Convictions”. There is a scene where G'Kar and Londo (who hate each other) are trapped in a lift car. The original script called for G'Kar to speak to Londo about his lack of sympathy for their situation in a cold and heartless tone. But for whatever reason, Andreas Katsulas (who plays G'Kar) delivered his lines in a completely different tone: that of Comedic Sociopathy, laughing like a madman at times. It completely puzzled J. Michael Straczynski at the time, but once he realized what happened, he realized that Katsulas had outdone the script. Not only did he throw it in (a rare event indeed), but he remembers it as one of his favorite moments of the series.”

— Live-Action TV / Throw It In - TV Tropes