The new screenwriter always has problems with dialogue because working within a visual medium he is always instructed to ‘show it, don’t say it’. This means dialogue must be pared to the bone and everything, even emotion, must be displayed if at all possible. If a character can let the audience know his feelings with a wink rather than verbal exposition, then the wink wins.
Write your dialogue as normal, if you must, but be aware that you’re going to have to go back and strip them back. This means that every word you use must be the right word and vital. Characters in a screenplay do not chat, everything must be driving the plot forward, revealing a character trait or making us laugh. But never write to let the audience know anything directly, there must be a reason for the exposition one character’s dialogue gives to another.
Go through every piece of dialogue and ask yourself whether you can transmit the information visually rather than with words. A kiss rather than ‘I love you’.
Always remember that dialogue eats up screen time much more than action.
Having said all that, it always surprises me that what audiences often remember from a film is a line of speech.