I often find it difficult to work a subplot into my screenplays. I get so focused on the main story that I end up leaving the subplot to the side and then it's underdeveloped. Any tips on weaving a subplot into the main narrative?
You can make subplots by using them to show how characters got involved in the main story. Try making a narrative that revolves around a single character and as an explanation of why they behave the way they do in the main plot.
If you've already made a plot and the characters are complete, try making a problem in one of the character's stories. After all, the subplot is merely a smaller scale with the same pattern that needs a goal.