You talkin’ to me, Zoltar?

The classic age-switch movie “Big,” released in 1988, made a star and an Oscar nominee out of Tom Hanks, who played the grown-up (in body only) version of a 13-year-old boy whose wish to be big comes true.

But the film, which receives a special 25th anniversary Blu-Ray release on December 10, almost took a very different road to adulthood, as Robert De Niro was once set to play the boy-turned-manchild.

The film, shot in and around New York City, was briefly set to star Harrison Ford at first, with Steven Spielberg directing — his sister, Anne Spielberg, co-wrote the film with Gary Ross — but the pair dropped out, and Penny Marshall took over as director.

Hanks was the first actor offered the role of the adult version of Josh Baskin, the boy who is granted his life-changing wish by a fortune-telling machine called Zoltar Speaks, by Marshall. But at the time, there were several other films with similar plots, such as “Vice Versa” and “Like Father, Like Son,” and “Big” seemed to many like a sure-fire failure.

So Hanks turned down the part, as did Kevin Costner and Randy Quaid. Sean Penn did a great read for Marshall, but was too young for the role.

Faced with these rejections, Marshall switched tactics. She decided to make adult Josh a real “man man, not a boy man,” and contacted her old friend De Niro.

“At the time, he wanted to do a commercial film, and he said yes,” says Marshall, braving the flu to call the Post to discuss her first directorial success.

To help him grasp this challenging character, De Niro told Marshall to watch all of his films, and highlight aspects of different characters that she wanted to see in Josh. She did so, especially noting his wildman energy in “Mean Streets” as a plus for Josh, and even worked with De Niro and Jared Rushton, the child actor who played Josh’s friend Billy, in her driveway.

But according to Marshall, the studio wouldn’t pay De Niro’s asking price. Producer James L. Brooks, Marshall says, suggested that Marshall give her salary to De Niro, and she was willing.

De Niro was not.

“He was getting a little annoyed,” says Marshall. “He said, ‘I don’t want your salary. I’ll take Jim’s salary.’ He’s not stupid, Bob. He said, ‘[You and I are] gonna be working on this. It can’t be that.’”

De Niro left the film, but his brief time on it made it a hot property, and Hanks, then shooting “Dragnet,” was convinced to make “Big” his next project. (Which is not to say that the actor was convinced “Big” would be successful. Co-star Elizabeth Perkins says that due to the similar films, she and Hanks “looked at each other at one point [during the filming] like, ugh — this is going straight to video.”)

Now, Marshall had to get Hanks into the childlike state of mind.

“The key words for Tom were ‘innocent’ and ‘shy,’” says Marshall. “Twelve-year-olds are not that outgoing. They’re not chatty so much. So I’d always say, ‘inch, [short for ‘INnocent and SHy’], Tom.’ No one else knew what I was talking about. I’d say, ‘Make it inch.’ And he was great.”

Marshall also videotaped David Moscow, the young actor who played Josh as a child, performing all of Hanks’ scenes in the film with the other actors, so that Hanks could use it for reference. Hanks then took this tactic further, videotaping Moscow and his friends acting out specific things that Hanks would have to do in the film, so he could see how children would do them.

There’s a scene in the film that was shot at the Thompson Street Playground (now Vesuvio Playground) in SoHo, where Hanks and John Heard, who plays Josh’s adult rival, roll around fighting on a handball court, Hanks playing keepaway with a ball by switching it from one hand to the other.

That move came directly from Moscow and his best friend, Earnest, who had been asked by Hanks to re-create the scenario.

“Tom said, ‘alright. You’re running after each other on this handball court. How would you keep the ball away from each other?’” says Moscow. “As childhood friends do, we probably had that issue come up a billion times at that point. And that’s exactly what we would do — switch the ball quickly from one hand to the other. [When the film came out], my buddy and I were watching that in the movie theater, because we hadn’t seen dailies, and we went, ‘Oh my god! That’s Earnest!’”

For Moscow, who was twelve at the time and got the role on only his second audition ever, filming Hanks’ scenes led to some awkward moments.

At one point, he ran lines with a casting director in Marshall’s office for the scene where adult Josh, in way over his head during a romantic scene with Elizabeth Perkins, touches her breast.

“That was one of the most embarrassing moments of my childhood,” says Moscow, who clarifies that Perkins was not there at the time.

“I remember sitting on the floor in Penny’s office, having to do that scene, and I was bright red the whole time. I’m not even sure I got all the way through it. It was a surreal moment.”

Moscow’s influence on Hanks’ take on the character was so great that it even affected the actor’s footwear.

“My feet were growing faster than my body, so I had a really strange walk, like a sloppy-feet type of walk,” says Moscow. “And I had Converse, which run large anyway. So Tom had these shoes made that were insanely huge for his feet, and during shooting, he ended up walking like I walk, which was this kind of duck walk.”

Once enmeshed in the character, Hanks was unflappable, rarely flubbing lines or breaking character, and improvising several scenes that became classics.

One took place at an office party where adult Josh, in a white tuxedo, makes a comedic meal out of the gross (to a kid) adult buffet food, spitting out caviar with over-the-top childlike facial expressions, and licking cream cheese out of a celery stalk like it was the side of a Devil Dog.

Perkins calls the filming of this scene, where Hanks improvised around 15 different bits, one of her most incredible moments on set.

“He walks through the door in a white tuxedo, and the first choice he makes is, ‘Everyone’s staring at me. Is my fly open?’” says Perkins. “That was all Tom Hanks. Penny is one of those directors who says, ‘let’s just do it again and see what happens.’ So there was an enormous amount of play, which really allowed Tom to find those adolescent moments that made you believe he was thirteen.”

Hanks’ improvisations came fast and furious. During a scene filmed at a loft on Grand Street in Soho where he and Perkins jump on a trampoline — a scene that “petrified” Perkins, since the trampoline was right next to a wall of windows — she says there’s one funny line in the film that many people miss.

“Most of that [scene] was improvised. We jumped on that trampoline until our bottoms were sore,” says Perkins, of filming in the toy-filled space. “There’s a tiny line you can’t hear very well. When I’m getting up on the trampoline, Tom says, ‘let me move my big balls.’ You have to really listen for it, but it is there. That’s the kind of thing he would just throw out.”

While each of the film’s scenes had their own unique challenges, the greatest might have been the movie’s central story arc.

With Perkins and Hanks, who was essentially playing a 13-year-old, falling into a romance, including strong implications that they had some sort of sexual contact, Marshall had to take great care in how she presented the material.

“To put these people in bed and actually have them have sex, where do you draw the line?” says Perkins. “She can’t have sex with a 13-year-old. And yet, the next morning when you see him, you get the impression [from his happy expression] that they slept together, or that he has been sexually fulfilled somehow. But it’s that very, very fine line of, the audience always knows he’s a 13-year-old, but the [other characters] do not.”

This made the film’s ending all the more complicated, as Perkins watches Josh, the spell now broken, return to childhood – and realizes for the first time that whatever happened between them occurred between her and a child.

“[The writers] did write at the end that when she drives him home, she kisses him on the lips. She knows he’s 13 by then,” says Marshall. “I said, ‘No no no. You can’t do that. [She] must kiss him on the forehead.’”

“Big” went on to earn two Oscar nominations – for Hanks, and for the screenwriters – and also became the first film with a female director ever to earn $100 million at the box office.

But the real trick to its staying power is that “Big” showed a generation the importance of staying young at heart.

“[Josh] brings out this innocence in [my character] that’s been shut down for a really long time,” says Perkins. “‘Big’ was all about everybody regaining their innocence.”