Here is a fascinating set of interviews with 10 great screenwriters from Tony Kushner (Lincoln), Lucy Alibar (Beasts of the Southern Wild) to John Gatins (Flight) and seven(!) other writers. They all talk about the scenes that were hardest to write for them. Very illuminating reading!
One of the hardest things to tackle in a scene is exposition. Exposition is the information that you need to give the audience. But it’s often not realistic for the characters to blurt it out. Rather you have to look at a scene from the character’s perspective. What do the characters want? What are their goals? And what are the obstacles to their goals? Obstacles are important. Without obstacles or conflict, the scene is dead.
If the characters behave realistically, then you can see if it can be natural for them to reveal the information that you want them to reveal. If not, then you might have to find another scene where you can do that, or pair the characters with other characters.
And dialogue? Dialogue always comes last for my. First I map the scene out. What do the characters want? What are they doing in the scene? Then, when I know what’s happening and what they want, the dialogue flows naturally from that.
It’s hard. That’s why even the best have problems doing it.