ROBERT DE NIRO is not the easiest interview in the business. Ask him a question that has nothing to do with his personal life, and he is likely to tell you that it is a personal matter. He'll answer some questions, but he is just as likely to shrug, look at his audience and bring a particular line of interrogation to a halt.
He is appearing in the new film ''GoodFellas,'' based on the book ''Wiseguy'' by Nicholas Pileggi, one of our leading authorities on the Mafia. Martin Scorsese directed. Scorsese also co-authored the script with Pileggi.
Scorsese and De Niro go back. They knew each other when they were kids in New York City. Later, they would know each other professionally, and the association has been a profitable one for both. Scorsese and De Niro have done six films together, beginning with ''Mean Streets'' in 1973.
The streets are just as mean, maybe meaner, in the new film, and De Niro was glad to have done it, even though he doesn't really have the lead. Ray Liotta does. He is Henry Hill, and this is Henry Hill's story.
At a interview in New York, De Niro admitted that the character he plays in the film, Jimmy Conway, may be similar to others he has done, ''but they are always different,'' he said. ''There are some similarities, of course, but they are fun to play.
There was never any question that he would do ''GoodFellas.''
''I'm proud of anything I do with Marty,'' said De Niro. ''Marty is always a special consideration. I just did it. I didn't question it.''
Why is the American public so fascinated with the Mafia?
''Well, 'The Godfather' sort of popularized gangsterism in a grand sense,'' he said. ''It was a movie about tradition. In a romantic sense, there was more loyalty in this family than there was in the government. If you had no faith in your own government, you could have it in this one.''
Did he research his role in ''GoodFellas?''
''All I did was talk to people who have had some relevance and connection to the character, and Nick was a fountain of information,'' he said.
Asked if the spectator is expected to root for the bad guys in a film like this, De Niro said he didn't think so but that ''in our society, the bad guys get publicity. There is a certain amount of glamour to them, but we must remain aware of what they represent.
'' 'The Godfather' romanticized the Mafia, but in one sense it was positive,'' he said.
One of the reporters said that De Niro is recognized as perhaps the greatest screen actor alive. What did he have to say to that?
''I don't look at it that way,'' he said. ''If you feel about yourself as others perceive you, you can get a little nervous. I don't really think about those things.''
He said that he and Scorsese didn't really get to know each other until the early '70s. ''I had seen Marty's first film, 'Who's That Knocking at My Door?'' and we spoke. He was about to do 'Mean Streets,' and we became grown-up friends. Today, we are good friends and best friends when we work together. It's a special relationship, but again, it's personal, private.''
So what does he give you that other directors do not?
''Well, we have a special way of communicating, a special working relationship. Marty is open to ideas from everybody. It's a pleasure to work with him. The more you come up with, the more he is excited, and it becomes a joy. It's hard work, and if you're working with people you don't respect, then it's not worth it.''
De Niro has won two Academy Awards, best supporting actor for ''Godfather II'' and best actor for ''Raging Bull.'' Does he value these citations?
''It's always nice to be acknowledged,'' he said. ''It's like a comparison. Sometimes, people who get them don't deserve them, but it's still nice to get them.''
The interview took place at the Saint Mark Hotel in New York. The evening before, the reporters had been introduced to the TriBeCa Film Center, the commercial complex De Niro owns in Greenwich Village. Would he say something about that?
''No,'' he said. ''I've said so much about it already.''
This time, however, he didn't say it was personal. By this time, he was speaking pretty freely for Robert De Niro, who always does his interviews in combination with others (his partner, this time, was Scorsese). He seems to prefer that. It helps keep the reporters at bay. Not that he would allow them to get too close. If they do, he can always say, ''too personal.''