If you are interviewing a guy like Steven Alfonso Collura it is best that both of you are inside a bank vault, because he has made a lot of enemies.

Collura was an undercover cop in New York from 1968 to 1973 who specialized in narcotics cases. He infiltrated the Mafia and set a one-year record with 126 felony narcotic buys, becoming one of the force's youngest gold-shield detectives, a high honor in the New York Police Department.

He is an actor by trade, but it was the back streets, the abandoned tenements, the waterfront, the places that tourists never see, that became the stage he chose to work on for five years.

"When you're born on the section of Union Street in Brooklyn where I was raised, you either go into Civil Service or the mobs," says Collura, who is 31 years old, single, 6-feet-1 and 70 pounds. He is a guy who could make a lot of women break dates to taste some wine with him.

He is also the kind of person who sits on the edge of his chair, moves his hands a lot when he talks and has a nervous energy that would make him feel uncomfortable relaxing.

Collura's father, Alfonso, was a hardworking man who got up at 5 six mornings a week to ride the subway to Manhattan to run a presser in a laundry.

He immigrated to the United States from Sicily, and married a second-generation Italian-American named Theresa with whom he had five children - Salvator, Phyllis, Rosemary, Dominick and the youngest, Steven Alfonso.

Alfonso became "Allie" and sometimes "Baby." And from the beginning he hated the association of the word "Mafia" with "Italian." "My father was a hard-working man and one of those Italians in the majority who were straight as opposed to those who were 'connected'," he said.

His brothers Sal, a longshoreman, and a construction worker, were big, strong Dom, a construction worker, were big, stayed away from the mobs. Like most younger brothers, Collura was looking for his own identity and hated to be called Sal's or Dom's kid brother.

There always seems to be more adventure down the street a little way from your own home, and one day Allie went to find it. Sharkskin

On the corner of President and Union Streets the sharkskin-suited, diamond pinky-ringed hoods from the Ballo mob hung out in their spit-shined shoes. It used to be said that a kid could earn a quarter there for starting a mobster's car in the morning.

"Lefty" Castiglione, "Rizzo" Rizzetello, Danny Longo, Richi "Pasts" Pastigletti were names that Collura would run across later in his police work, and were also names that would be chiseled into gravestones early in their careers.

The area was considered dangerous but alluring, so Allie at the age of 10 wandered down and became one of the trusted "gofors," running for the newspapers to get the daily racing line.

Messages were sent to Gallo headquarters, and every once in a while even "Crazy" Joey Gallo himself would show up on the corner. For Collura, the highlight would be when "they laid a dollar on me for just getting a container of coffee for them."

"Did you ever have a dollar in your hand when you were a kid?" he asked. Then he held out the palm of his right hand and patted it. "A dollar, right here. If my father or brothers knew I was working the corner they would have whacked me all over the place."

When Alfonso Collura managed to save $2,000 for a mortgage the family moved to a house "with a little backyard in Bensonhurst, on the other side of Brooklyn.

After graduating from New Utrecht High School, Collura enrolled at New York Community College, transferred to Brooklyn College, then left to do a six-month stint in the Army Reserve.

When he returned, he was undecided on what course he would follow until he met a model named Lisa, and began to follow her.

Lisa was a drama student at NYU, so Allie enrolled and the two later took acting lessons at the Herbert Berghof Studio in Greenwich Village. But to live high in the style of his idol, Joe Namath, cost a lot of dollars that Collura didn't have.

Almost by accident he found out that he could bowl. He teamed up with Johnny Petraglia, who is now one of the top professional bowlers on the national television tour. The two took on all corner, had a lot of money bet on them - sometimes thousands of dollars - and they lived off the percentage of the backer's winnings.

"I was a hustler, making good money, contributing at home and beginning to really get into the acting business." Discovered

The Royal Box Theater is a small playhouse on Second Avenue in the East Village.

Collura landed the roll of Johnny Pope, the junkie in "A" Hatful Of Rain."

"Hatful" was originally a Broadway play, later a movie, and the part of Pope had been played by Tony Francios and Ben Gazzara.

Collura was good in the role, so convincing as a junkie that one night while performing he spotted a narcotics detective named Mike Asti in the audience.

Asti had been in the papers a lot and was bad news to the pushers and junkies in the city. He was also a next-door neighbor from Bensonhurst.

When the theater emptied out, Asti remained, and when Allie had to leave he had no way out but to pass right by the 40-year-old detective. Asti smiled and said, "Come on kid. Let's go to Vincent's for squid and wine."

The restaurant on the corner of Mott and Hester Streets is a popular hangout for the theater, sports, and other types, and Allie quite often went to eat and look.

When they finished the mean Asti told Allie that he was a good actor, and also told him that other narc detectives had been keeping an eye on him. Then he startled Allie by asking, "Tell me the truth, kid. Are you using?"

Collura said he wasn't, had never even smoked grass. But like a good cop, Asti moved in for a couple of quick jabs, describing the performance on stage as being too expert for a guy who hadn't been around the stuff. Asti talked about undercover police work, told Collura he had only been testing him and that if he wanted to meet the next night he would show him where the real drama was, "with no curtain calls." Undercover

Collura met Asti, who was wearing an army jacket who sizes too big for him, and they took off for 125th Street near Lenox Avenue.

It was a tense scene Asti spotted a group of tough blacks, talked about Riker's Islad, dropped a couple of names and made a "buy" - two nickel ($5) bags of heroin.

On the way downtown Asti talked on his police radio to a pair of detectives who had observed the purchase and made the arrest.

It was about 3 a.m. when they drove toward Little Italy in the Village and Collura blew his top at Asti, telling him that they could have been killed, but at least Asti had a gun and he didn't have one.

Collura slept very little during the next 48 hours, but he did a lot of thinking about Asti's words "You're a cop, kid," Asti had said. "You'll be a good one. We need people for the undercover unit, we need help."

The pair met again and Asti said, "I want you to give me three hours of your time, no matter what your decision is. They're giving a Civil Service exam Saturday morning at Thomas Jefferson High School in Brooklyn. Will you take it?"

Collura, now feeling a strong sense of responsibility, shouted, "I'll take it, I'll take it. Now tell me what the . . . this is all about?"

He found out the hard way. He was badge Number 6178, a white shield that made him a private in an army of over 30,000 police officers, but he was one of 25 that onl ya few fellow officers would know about.

Collura went through the academy alone, with special hours on the firing range. He answered to only one superior officer.

His family could not be told nor his girl friend, who later dumped him because of the eccentric hours he kept and his inability to explain to her where he had been or where he was going.

The world he moved in was populated by junkies, who had been "turned around" by cops to point out pushers, and other junkies whose only pay was a few more days out of prison for every "score" they made for the narc cops.

There were pimps and prostitutes. Two prostitutes he had become fond of and used a lot in police work were seen walking out of a hotel with him by his girl friend, who had followed him one night after he told her he had a meeting with his producer. She left him that night, and he could never tell her the truth - that he was using them that night on a stake-out.

He infiltrated a tough Puerto Rican street gang and was knifed in a street tight. He was arrested by cops while making a "buy" in a doorway, had his head busted against a wall and was dragged off in handcuffs, still not able to reveal his identity.

During this period he was still appearing in a daily soap opera called "The Doctors" and had gotten a small part in the movie "Midnight Cowboy" in which he sold some junk to Dustin Hoffman. Gunfire

A contact he worked with stumbled onto a connection bigger than the "French Connection," a shipment of drugs about to land in Brooklyn from Marseille. The contact, "Frenchie," was a turned-around junkie and good informer. He worked hard and became a police buff. But Frenchie made a mistake.

He asked a Mafia type the wrong question, and Collura found him a few days later in an abandoned tenement. He had been beaten badly and forcibly given a "hot shot" of Heroin that killed him. Collura became enraged.

The famous "French Connection" case involved 112 pounds of nearly pure heroin that would make about $32 million on the street. The shipment that Frenchie, had stumbled onto was believed to be 220 pounds and worth about $70 million.

One night Collura went into a Harlem flat to close the deal for the shipment. He had a tiny mike behind a cross hanging from his neck, and was backed by a team of detectives.

The deal went sour. There was gunfire. Collura was wounded on the back of the head. When the team of detectives monitoring the voices busted into the flat there were two dead hoods on the floor, and two had escaped.

There were no arrests made in the case. Collura was decorated in a quiet ceremony, but he was disappointed that even his mother couldn't show up.

Then he was taken off the narcotics squad, promoted to a detective sergeant in homicide. Collura didn't like the idea, so he resigned from the force. Curtain Calls

There is now a book, called "Collura - Actor With A Gun," written by Bill Davidson, about his exploits. Collura thinks of the book as "A love affair that an Italian kid has with his country."

He calls himself "pro police: I'm no Serpico, he ruined a lot of good cops.

"The book is not a cop story," it's about a kid who could have gone one way or another."

There has been some interest for a movie, but Collura feels he must play the lead. "I want to show a very sensitive movie."

Collura has formed a group to tour schools and lecture on an anti-drug program.

He still has a few TV bit parts to hold him together, and hopes the book will sell. His plan is to donate part of the proceeds to the Widows and Orphans Fund of the NYPD.

"I am very proud of being Italian, and tell it to everyone. Italians should stick together."

All you could feel as he walked away was that for a 31-year-old. Collura has done a lot of living so far.