Downey's Hank Palmer has, like many of the actor's previous characters, the gift of the gab, something he uses to withering effect on those around him.
"I love that he's such an a**hole," Downey smiles. "I love this story; it's so entertaining because when I sit and watch this movie, the audience is laughing their asses off at different times, but I think they are doing it because after things get a little heavy, they are like, 'I need to recover from everything I just saw.'"
As Palmer, he must defend his estranged father in a vehicular homicide case.
"The movie is about fathers and sons," says Downey, "but I think it's about a lot of things including how you can get pulled into something out of an obligation and how it can then turn into something else. You can get sucked into things. And on the other side of that, you gain something you would have never got because you tried to understand someone rather than just have everyone try and understand you."
On the subject of the parental-child dynamic, how does he get along with Downey snr these days? His father's drug-addicted lifestyle took a toll on his son which began when Junior, aged 6, was handed a joint by his father. Now 49, he would go on to become as famous for his widely documented battle with drugs and alcohol as he would for his career-defining roles.
"We get along great now. Time is short," he laughs. "Somebody said to me that their shrink once said to them, 'You don't need to have a relationship with your parents, you need to manage your parents'. Maybe this guy was completely right."
The Judge is very much a family affair. It marks the first production from Team Downey, a husband-and-wife collaboration.
He consistently defers to his wife, Susan ("The Mrs") whom he met in 2003 when she was producing Gothika, and who is largely responsible for his sobriety. After a two-year engagement, they married in 2005 and are now raising a son, Exton, 2, and "The Mrs" is six months pregnant with a little girl on the way.
A working relationship with one's spouse must be rather precarious at times? "Well, she's crazy," he jokes. She is most certainly the straight guys to his various eccentricities.
"Look, I got really lucky. Before she was Susan Downey she was my buddy, and she still is.
"She's a great mum and it never ceases to amaze me the amount of patience she has for complicated people.
"But I don't know a couple that doesn't fight. Some couples definitely shouldn't work together; they probably shouldn't even vacation together."
Budgeted at US$50 million ($64 million), the film is directed by David Dobkin. It's his first drama after comedies The Wedding Crashers and The Change-Up. The cast includes Vera Farmiga, Billy Bob Thornton and Vincent D'Onofrio.
Despite the impressive ensemble the movie is dominated by Duvall and Downey. Downey found it refreshing working with the veteran.
"Duvall will say, 'I don't want to talk for five minutes about the scene we're going to do. In fact, don't say anything; let's just do it.' I think of him as the 'Nike' of actors 'just do it'."
Vanity Fair recently named Downey as the world's highest-paid actor. This declaration was gleaned from Forbes Magazine's report that he drew in US$75 million from the backend profits of Iron Man 3.
He sighs. "I am so f***ing bored with that subject. I remember living in an apartment and when you don't have the rent for the next month, all that matters, is, 'How is salvation going to occur, how am I going to get six hundred bucks?' And that's the only problem that money solves. It only takes away fear of financial insecurity and fear of financial insecurity is one of a thousand fears. But it's definitely a big one.
"Let me put it this way. I think it was Harrison Ford who said this: Money is important until you have it."
