The key to writing a great novel or screenplay is creating powerful, vivid characters. Think of your favorite film or novel from years past. What do you remember? Probabiy one or two key scenes but, above all, a character that has stayed in your memory.
The ones that come to mind for me include Lawrence of Arabia, the Godfather, Gloria (the Gina Rowlands version), and Hannibal Lector. What did they all have in common? They were bigger than life. Sometimes they're that way going into the story, sometimes it's the events of the story that brings out something special in them.
The better you know your characters the easier you will find it to write their dialogue and to know what they would do in any situation, which certainly helps with plotting. Here are some unusual ways to explore your characters:
The Eulogies
Imagine one of your main characters has passed away. What would each of the others say at that person's funeral if they were honest about their feelings? Do this for each of your key characters.
The Party
Imagine you meet your character at a party. What would they be wearing? What would they talk about? What would they be trying to hide? What would they be drinking? Would they drink too much? Talk too much? Join in if people started dancing? Who would they want to talk to? Who would they want to avoid?
Time Machine
Imagine that you could send your character back in time to the most significant (good or bad) day of their life. What day would it be? What happened? What was the effect at the time? What has been the lasting effect?
Have fun getting to know your characters, I think you'll find it stimulating and it will enrich your writing.
(Would you like to find out what writing advice was given by the greatest classic and modern authors? People like Dickens, Austen, Hemingway, and Vonnegut? It's all in my book, Your Creative Writing Masterclass, pubilshed by Nicholas Brealey and available from Amazon or your other favorite bookseller.)