Eight years ago, even before he wrote and directed Boiler Room, his tense little coming-of-age drama about unethical stock brokerage, Ben Younger had this idea for a comedy.

"My mother’s a psycho-therapist," he says. "And my girlfriend, at the time, was in therapy. I thought, `What would happen if she turned out to be my mother’s patient, and she didn’t know it?’"

Prime is the movie that came from that. Younger, 32, made his second feature film about an older woman (Uma Thurman) rebounding from a divorce with a younger man (Bryan Greenburg) whose mom (Meryl Streep) turns out to be the woman’s therapist.

"In New York, this must have happened," Younger says.

Working his way through the moral and ethical concerns of such a relationship was tricky. But Younger knew he could get away with it when he landed his dream cast – Streep and Thurman. And the scene in which the psychiatrist-mom tells the girlfriend-patient that the man the girlfriend has been baring her soul about – sparing no sexual detail – is the shrink’s son, is where that casting paid off. The two actresses flash through moments of embarrassment, dismay, anger, humiliation and revelation in just a minute or so of screen time.

The movie’s reviews haven’t exactly sung (Variety used words like "underwritten" and "mediocrity" to vicious effect). But everybody is noting "Streep and Thurman’s obvious enjoyment in playing off each other" (Variety as well).

Here’s how the big scene came about.

"Sometimes, a scene works too perfectly," Younger says. "You have time to try it another way. You’ve hit the three-pointer. Why not try a dunk? If they nail it on the first take, why not try it differently? If it’s sad, make it happy. If it’s happy, make it melancholy. Why not, if you have the time?"

That’s what Younger did. The scene Younger wrote was to be played straight, almost tragic. But it ended up comic.

"We explored that scene more than any other one we shot," he says. "We did it on set, not during rehearsal. Take after take, starting one place, a very serious read on how Lisa (Streep) tells Rafi, and how seriously Rafi reacts to that, to the less serious and less serious. You see what feels good, and you take it in that direction. And gradually, it became funnier and funnier and more and more subtle.

"You don’t tell the actors, ‘That’s not working.’ You just keep saying, ‘That’s great. Let’s try it another way.’

"That’s the nice thing about working with Meryl and Uma. You can make that change without them thinking it’s some dramatic or scary event. They go, `Oh yeah. You want to try it this way? Sure!’ A 180-degree turn in the intention of the scene, how they play it and what it says, and it’s nothing to them.

"The editing style of the movie is odd," Younger says. "People ask me, ‘Why is your camera on people who aren’t talking so much? That’s not the way it’s done. You watch the person talking.’ But I kept it on the actress not talking because the reactions are better than the lines."

© 2005, The Orlando Sentinel (Fla.).

Distributed by Knight Ridder/Tribune Information Services.

Prime is currently in theatres.