Ten of Hollywood’s top screenwriters shared their secrets with their writing brethren Tuesday — with a major focus on the precision needed to create believable characters such as Jesse and Celine in “Before Midnight.”

“The script was obsessively written, down to the commas,” said Julie Delpy, asserting that not a word of the romancer “Before Midnight” was improvised.

Delpy appeared during the Writers Guild of America’s “Beyond Words” panel discussion at the Writers Guild Theater in Beverly Hills. Writers from seven of the 10 screenplays up WGA awards attended the two-hour event, which drew about 400.

“We’re giving it rhythm during the writing process, which is really the hardest part of this process – to do the final film when you’re actually writing it,” she explained.

Delpy said that a 14-minute car scene — shot in a single take by director Richard Linklater — was by far the most difficult in the film.

Delpy, who is up for WGA and Academy Awards in the adapted screenplay categories with Ethan Hawke and Linklater, said the trio spent years constructing the back story of the Jesse and Celine characters from 1995’s “Before Sunrise” and 2004’s “Before Sunset.”

“Only when we figured out what happened did we start writing,” she recalled. “We want to know everything that’s happened with them over the last nine years even if it’s not in the film.”

The discussion, moderated by WGA West VP Howard Rodman, often touched on writers being inspired by real-life examples to create characters such as Jennifer Lawrence’s Rosalyn in “American Hustle.”

“Many times I’ve had my ass handed to me by someone like that,” said writer-director David O. Russell. “I love creating characters with flaws.”

Tracy Letts admitted that part of “August: Osage County,” which he originally wrote as a play, was based on real-life experience — particularly Meryl Streep’s cantankerous Roberta character.

“That’s my grandmother,” he said. “She was capable of monstrous things.”