That quote is from Harrison Ford, and it's something he said to George Lucas with regard to the clunkiness of his dialogue. It's a lesson that writers all too often forget.
One of the cooler aspects of the filmmaking process is getting to see casting sessions. Through my own short films, and the films which the companies I've worked for have made, I've gotten to see several casting sessions. (Obviously in the case of the professional films, I've only seen the auditions on tape, whereas I was actually in the room for my own.) It's interesting to see different actors make wildly different choices, and just as interesting when disparate takes on a character are equally valid.
As writers, sometimes we convince ourselves that there's only one way a line of dialogue could be said. Maybe we assume that there's only one way a line could be delivered, or convince ourselves that a potentially difficult line could make sense if spoken just precisely. But here's the thing, actors will surprise you. They might find subtext you didn't realize was there - or they'll pick up on subtext elsewhere and find a way to bring it out using a line you hadn't intended for that purpose.
On one short film, I'd written a character who was supposed to be rather narcissistic. In my head, she was seeing everything through the lens of how it affected her and what it meant for her. Her interactions with the lead backed that up, but as I saw it, her self-centeredness would be almost naive. No matter what you said to this girl, she'd bring it back to herself because that was how she thought.
Well, the actress who eventually won the role came in and played those lines with more ego than I had imagined. In her interpretation, the character knew she was hot shit and she played her dialogue far more self-aware than I had conceived it to be. It surprised me because I hadn't considered that take on the character, but the more I saw it, the more I liked it because it made for a more interesting conflict with the lead. Though that dynamic wasn't the central conflict in the script, it put the lead more on the defensive and that made for a more interesting short all around. Had I gotten an actress who had given me exactly what I wanted, I think I still would have gotten a good movie about it, but this new spin on that character certainly enhanced the film.
But sometimes actors will take dialogue the other way. You hear it outloud and you realize that it simply doesn't work. There's nothing more punishing than having to endure an actor deliver a terrible line reading as you realize that it's all your fault for writing that insanely terrible dialogue.
Characters should be living, breathing people. They're not like text-to-speech programs that can spit out anything that you type. Make sure your dialogue reinforces that.
