And even when he slows down to answer questions, the cell phone is still fully operational – and his mind is all over the place.

Obviously, he’s thinking about Alaia’s grand opening, and especially about all the celebrities and even childhood friends who have been there. Then there’s his movie “Friends and Lovers,” which opens Friday. And some upcoming movie roles, including Barney Rubble in “The Flintstones: Viva Rock Vegas.” Then he’s back to thinking about food and menus – and making nice with the neighborhood cops.

Billy Baldwin says the pace is typical for his little bro.

“He’s the more fun-loving, goofy one of us,” Billy says from a window seat overlooking the bar. As he talks, Stephen is dragging on a cigarette, occasionally waving in the air to add emphasis to his gab. Smoky circles fill the air. His ice-blue eyes are dancing.

“He’s more willing to take a swan dive – a flying leap into anything he does in life,” Billy continues. “He’s the kind of guy who’s on a cliff in Mexico, about 150 feet up, and on the first take, he’s there, just going for it. Whether it’s over the top, or a little over the top, or right on. I don’t think he has less fear than me. He’s just less timid, and maybe he’s less inhibited in some sort of way.”

Of course, this makes perfect sense. The self-proclaimed “runt” of an “incredibly rowdy, rambunctious, middle-class, Irish-Catholic, South Shore Long Island” family of eight, Stephen just doesn’t do timid.

He’s the youngest of four boys and two girls – the baby of Carol and Alexander Rae Baldwin’s family, which means he had to take chances if he wanted to do ordinary things like eating. And later, living with his siblings’ fame.

There wasn’t a lot of money in the Baldwin household – not with six kids – but there was a lot of love. And respect. That’s what shaped him, the 33-year-old Stephen says. His brothers and sisters too.

“My dad made $36,000 a year – had six kids,” he says. “It was a lower-middle-class kind of reality, and I’m glad to have that knowledge and history to reflect on. You know, when the water isn’t working in my trailer once in a while, I just laugh. I don’t really get too bent out of shape. It doesn’t mean I don’t do the best I can as a businessman, but I think my dad is looking down – really cracking up, laughing.”

Stephen followed his brothers into show business – Alec went first, then William, then Daniel. He started by singing choral music and opera music at Massapequa High before being accepted by the American Academy of Dramatic Arts. When an agent finally spotted him, he was working in a pizza parlor and modeling for Calvin Klein.

He landed a few small film roles – “Homeboy” (1988), “Last Exit to Brooklyn” and “Born on the Fourth of July” (both 1989) – and TV gigs like “The Young Riders,” which ran from 1989 to 1992. Soon he was getting meatier work like “The Usual Suspects” (1995), but he was generally cast as a doofy, irresponsible guy – “Posse” (1993), “Threesome” (1994) and “Bio-dome” (1996).

To some extent, “Friends and Lovers” is more of the same – light on doofy, but heavy on irresponsible. It’s set during a ski weekend of sex and romance among long time buddies; Baldwin plays a sex fiend intent on bedding anything with breasts.

Although Billy says that “90% of what you see” in an actor’s performance is “who they are in real life,” Stephen insists that he’s nothing like that. “Well, in high school, maybe. But I met a great lady who whipped me into shape.” he says, beaming.

The lady with the whip – or as Stephen terms it, “the Brazilian whammy” – is Kennya Baldwin, a graphic designer he met on a Manhattan bus in 1987. He relentlessly pursued her, going so far as to sleep on a bench at the entrance to the Parsons School of Design, where she was studying.

“I was like a real bad guy – trouble,” he says, giggling. “I think it was a combination of my looking for an angel, and her looking for a little trouble.” They married three years later, and have two daughters, Alia 6, and Hailey, 2.

This year, Stephen approached Billy with the idea of opening a restaurant right across from where he met Kennya, at Fifth Ave. and 13th Street. The spot, which had been renovated just before its owners at the time had gone out of business, was a steal, Stephen says – and he had an awesome group of investors behind him.

But Billy, one of the more cautious of the brothers, raised an eyebrow when he heard the idea.

“Everything that happens among brothers is like some kind of a NATO covert operation,” Stephen jokes. “We’re flanking from this bunker, doting the projections, trying to figure it out. And, of course, when the runt gets an idea . . . “

Billy finishes his brother’s sentence: “Automatically, it’s a problem. You know it’s gotta be some scheme or dream or some harebrained operation.”

But the “harebrained operation” is gorgeous – a stucco-styled American/Mediterranean bistro that’s already attracted the likes of Jerry Seinfeld, Elton John, and Liam Neeson.

The brothers insist that Alaia (pronounced AH-lee-ah – it’s a variation on Alia) isn’t just for the rich and famous and hungry.

“We don’t want to run a place where, if Jerry Seinfeld comes here, we’re going to call the paper and say, ‘Do me a favor – put his name in the paper,’ ” Stephen says. “We want the kind of place where everybody who comes in from Massapequa and says, ‘Yeah, I know Alec Baldwin,’ to know they’re welcome here, too.”

The bros from Massapequa wouldn’t have it any other way.