1. Use as few words as possible.
I taught myself this on GoAnimate because I realized early that using speech bubbles instead of actual voices would slow the action down. You want people to be able to read your dialogue as quickly as possible so they don't become aware they're reading. This way they will still feel like they're watching a video and not reading an animated comic.
I later read in a book on how to script write for animation that professional animation script writers also aim for telling their stories in as few words as possible but for different reasons.
In animation it is still quite hard to lip sync a character to the dialogue. The longer you watch a single camera shot of a character talking the more aware you become that you're not watching a real human talk. You start to notice inconsistencies.
To get around this professional script writers change the camera view often, never allowing a character to say too much in a single shot.
Even better, if they can get a character to say dialogue off camera they will because, not only does this mean no lip syncing, it also allows for minor changes in dialogue after the drawn component of the animation is complete.
Hence you should review every line of dialogue and ask yourself,"Can the character say the same thing with less words?"
If you find you can't cut any words out then break the talking up over different camera shots to keep your scene visually interesting whilst delivering short bursts of dialogue in each shot.
2. Show don't tell.
Animation is a visual medium. It allows you to do so much more than film (excluding realistic 3D animation from the 'film' category and bringing it back to 'animation'). If your animation is just a bunch of talking heads you may have well as done it with film.
That said, if you follow tip 1, then you'll quickly realise the way to use less dialogue is to show more of what your character would have described in the dialogue before you trimmed it down.
Showing the action will also be more interesting to watch than long scenes of characters talking.
3. Off camera action is your friend.
Also as mentioned in tip 1, off camera dialogue is favoured quite a lot by professional animators and script writers to avoid lip syncing issues. However, off camera is also great for 'suggesting action'.
How important is it for the viewer to see an actual action performed? If it's not too important you can save a lot of animating time by using the off camera technique.
For small actions you can use the 'distraction' method where, for example, you focus the camera on one character talking whilst another character performs a simple action off camera. I give a more detailed explanation of this in my behind the scenes post for my Domokun animation, Tashanna and the City.
A simple example might be a scene in a bedroom. One character is standing and the other is in bed. You want the character in bed to get out of bed and stand but you don't want to animate it.
You could have the character in bed say "I'm getting up now" in shot. The next shot is of the standing character saying "Good because we don't want to be late.". In the next shot the character in bed is now sitting on the edge of the bed.
This technique works because when we view things in real life we know things can happen out of our range of vision. Be careful though, not to do too much off camera.
If, in the previous example, in the final shot, the character in bed was now fully dressed, standing and ready to go, it wouldn't work because we know he didn't have time to do all that in the time it took for the first character to say "Good because we don't want to be late".
For much more complex off screen action you can use a variation of the distraction technique - focusing the camera on other characters viewing the action - then use audio to suggest the sounds of what's going on off camera.
For example you have a fight scene but you don't want to animate the entire fight on camera. To break it up you could show people watching reacting to the sounds of the fight in progress (by doing this you can actually suggest the fight is much worse than anything you could show on camera).
Hopefully these three tips will help you when it comes to writing your own scripts. I've found the last one particularly helpful when I need to improvise something that would be too difficult to animate on GoAnimate.
If you have any other suggestions/tips for writing better scripts please feel free to leave them in the comments below.