I've struggled with it. But it gets easier with practice.
A lot of conversation is redundant. Studies by psychologists have found that only about 5-10% of what we say is needed to understand the context. Everyday conversation, when put on the movie screen, seems to be stretched out.
So rather than stifle yourself, write out the conversation as you hear it. Then go back and pick out the key idea(s). Then hack it down to the essentials and use visuals/actions to help carry some of the meaning.
Code:
DAVE is standing beside STU behind stage. The crews are moving
props about.
DAVE
So you're telling me that Janet knew all along that
I was behind the curtain and could hear her! Why
that wicked little .... She could have stopped this.
STU
Hey, I'm just telling you what I heard, man. You know
how Gina likes to run her mouth. Well, I heard her
talking on the phone to Roxy telling her all about
Janet's little scheme.
So I just freewrite what is playing out knowing it will undergo revision later. Too many new writers want it right the first time. It just doesn't happen. The first rule of writing is that there will be a rewrite. (Even after a 'finished' script is optioned, it is likely to experience a re-write!).
Okay, ouch that dialogue segment hurts. It's LONG and comes across as talking heads. Now I need to revise it. So what is the real point to the dialogue above? -- Janet knew. What part of this could be shown visually? Stu seems a bit intimidated. So we will be ruthless here.
Eliminate progressive tenses from the actions/descriptions--no "is standing" or "are moving".
Separate actions by different characters and descriptions.
Look at implicit ideas--the character says "I hear", "I see" that would be obvious. [If he was behind the curtain OBVIOUSLY he could hear her. He doesn't need to say that.]
Show rather than Tell--Dave is angry so have that come across.
Code:
Stagehands move about props.
Dave advances towards Stu backstage.
DAVE
Janet knew I was behind the curtain! That wicked little ....
She could have stopped this.
Stu backs away and throws up his hands.
STU
Hey, I'm just telling you. You know how Gina runs
her mouth. Well, she was on the phone telling Roxy
all about Janet's little scheme.
This is much improved. It gives me a visual reference. Obviously if Dave is behind the curtain, he can hear.
Most of Stu's words were filler. With some editing, we cut from 63 words to 40 and made it more visual. The other side, is that shorter lines are much easier for actors to remember.
Good luck.