Screenwriting : Dialogue by Wayne Taylor

Write it the way the character will say it. You can write it in proper English at first and then rewrite it to reflect the character's voice.

As a Writer and an Actor you should write the way that the Character will say it. You however don't specify on accents or dialects unless it's part of the Character. The Actor will bring there own part into it. But when you writing if you write in proper English than people who read your script will get confused. Dialogue is part of Character building and not everyone speaks with proper grammar and English so it is important.

I'm an actor, writer and reader, and I find it's important to write in the way the character will speak... I feel it is better for all parties involved in getting the film made - from the reader who has to get a sense of the film immediately in order to want to move it up, through the producers, director and actors who have to grasp that same sense in order to bring about a cohesive production. Street-talkin' gangsters can't be written in proper English or the piece will fall apart. When writing highly stylized pieces now I'll even use colloquial terms in the action passages to keep the feeling moving. Also I find proper English rarely spoken, so as an actor, when I find it in a script I work to find the writer's reasoning for it's use... I remember being told by a science-fiction casting director that he often seeks classically trained and shakespearean actors for alien and inter-dimensional beings as they speak in proper English and the actor's ease of use of "I do not" over "I don't" etc, helped keep filming moving...