When I read a script, it can take a while for me to figure out if the characters are really fascinating and original, and if the plotting is satisfying, and if the stage directions reveal an engaged and engaging writer.  I usually need to read several pages to get a feel for all of that.  But I can tell right away if the dialogue is bad.  Other than something like a typo, this kind of error is probably the first one that your readers are going to notice.  It's also one of the easiest to avoid.  A thirty-minute pass through your script can probably eliminate the worst examples of unnatural dialogue writing.  Here's what to look for and how to fix it:

Stilted Dialogue.  This is usually a result of trying too hard to squeeze character out of every line.  Let's say your character is a rich businessman.  When he enters a meeting with a longtime business foe, you might be tempted to have him say something like: "Gordon.  Please, take a seat.  I think you'll find the room sufficient for your comfort.  I hope that phalanx of lawyers you've brought with you is an indication that you're ready to actually negotiate this time, not just waste both of our time with needless delays." 

It certainly conveys power, confidence, education, authority.  But do you really believe that's how these people talk?  It's so long and elaborate.  For one thing, it's way too long for a speech that doesn't change anything.  For another, a powerful guy doesn't have to speak in ways that are designed to make him sound powerful.  He knows it, the other guy knows it and the audience knows it.  Take out all of this stuff.  In general, make everyone say less.  It's hard to make "Take a seat, Gordon" sound stilted, and it's all the stuff that happens next that actually does the work in the scene. 

A subset of this problem is trying too hard to make very young characters sound young or foreign characters sound foreign.  A light touch is sufficient.

Glib Dialogue.  At some point you'll probably have at least one character speak from the heart.  They'll blurt out a confession of love or guilt.  They'll profess a belief or reveal a desire.  It's tempting to give them eloquence at this point.  They may suddenly declare:  "Don't you see?  I'm in love with you!  The reason I hid the ring wasn't because I was ashamed of you, it was because I loved you so much that it scared me.  Even back then!  Even back then when my heart was my own." 

You probably loved it while you wrote it.  You could feel the emotion and poetry in it.  But when you reread it, it seems glib and overwritten.  If you take the poetry out, it feels flat.  In my opinion, the only thing wrong with the line is that it defies human psychology.  We don't get articulate when we're emotional -- the opposite happens.  We get stumbly and tangled as we choke back our tears.  I might write this same declaration in this way:  "Don't you see?  I hid the ring-- I wasn't ashamed of you-- God, no.  I'm --  I did it because I'm in love with you.  I know you probably don't-- I don't want to scare you, but even back then.... even back then I loved you so much."    The poetry is gone, but so is the glibness.   It runs like a three-legged dog, but it's so much realer.  And you can let the poetry creep back in when you write the next line, after the heat of the moment has passed.

Familiar Dialogue.  Generally, this takes the form of jokes or quips that have been used before.  For example, any comeback with "much" in it (e.g. "Immature much?") has simply been used too much at this point to have any freshness left in it.  Expressions that felt amusing when you first heard them have a very short shelf life.  Referring to the early morning as "o-dark-thirty," for example, is hilarious the first time you hear it, and fairly deadly every time after that.  Remember that these expressions weren't always part of our quip arsenal.  Someone made them up.  That means that you have the power to make new ones.  If you've already heard it, do not use it.   Put some time into finding new expressions, and it will pay off for you in a big way.

There are more dialogue problems than just these three.  Make sure you also look for speeches that are too long, words and grammar that are too highbrow (or lowbrow) for the character using them, and characters who tell each other things they both already know.  You might also be surprised at how often characters say the same thing twice in a row.   I am constantly removing redundant sentences from dialogue.  Even "yes" and "no" can often be removed from dialogue, since they're almost always followed by a sentence that implies the "yes" or "no" on its own!

Even if you only have hours before you have to turn in a script, a quick dialogue pass can help your script start to stand out within moments after the reader picks it up.  Pull the weeds from the garden so the flowers stand out.

Jane Espenson is a television writer and producer. She has written for shows including Ellen, Star Trek: Deep Space Nine, Gilmore Girls, The O.C, Angel, Firefly and Dollhouse among many others. She is especially proud of her work on Buffy the Vampire Slayer, Battlestar Galactica and Caprica. She also writes comic books, webisodes and the occasional tweet.

Note from Headquarters: Jane's blog (though she doesn't add new content anymore) is packed with wonderful information. Check it out: http://www.janeespenson.com/