“Bad movie dialogue speaks in complete sentences without any overlapping or interruption, and avoids elliptical speech which is truer to how people actually speak.”
Of course you don’t want to write dialogue exactly the way people speak. The characters would come across as half-wits. If you are up for an interesting experiment, record a normal conversation and transcribe it. You’ll be amazed at the number of interruptions, sentences that change direction mid-way or just trail off, and non sequiturs.
For examples of the other extreme watch a bad soap opera. In these, characters tend to explain everything in complete sentences, tell another character what that person would already know, and helpfully recap incidents that took place in episodes the viewer might have missed.
What we’re aiming for is somewhere in the middle.
If you can find an image that can take the place of a long speech, so much the better, whether you’re writing a novel or a screenplay. An example comes to mind from a TV movie script I wrote some years ago. The protagonist goes into a bar and encounters a teacher he’d admired when attending college, twenty years before. The teacher looks down on his luck and is drunk. The protagonist asks him what happened.
In the first draft I gave the teacher a long speech about how he’d turned to alcohol and at first it seemed to help him but then he lost control of it, etc.
In the second draft I just have him lift his shot glass and say, “This. This is what happened.” Now that I think of it, maybe he didn't need to say even that.
(For more tips on writing good dialogue, creating vivid characters, and crafting strong plots, see "Your Creative Writing Masterclass," published by Nicholas Brealey and available now from Amazon or your other favorite book seller.)