In "Kramer vs. Kramer," scheduled for release next month, Dustin Hoffman plays the role of Ted Kramer, whose wife, Joanna (Meryl Streep), leaves him and their young son early in the story. Coincidentally, when he began working on the film last year, Hoffman was a newly separated father.

"I had not done a film where my life circumstances matched what I was doing professionally," he says, "and I enjoyed it." He laughs before adding, "That's perverse. But I enjoyed it because it's what I always wanted to do. It's what any actor wants to do."

Hoffman and Anne Byrne, a former ballerina who appeared in Woody Allen's "Manhattan," have been separated for over a year. Married in 1969, they have two daughters, Jenna, 8, and, from Anne's first marriage, Karina, 13.

"Statistically," he comments, "today your first marriage is a long shot . . . Is it not difficult to live with someone for 50 years and interact in a way that fulfills both of you?

"I'm not sure it's good to see two people who are 70 years old and interchangeable." He thinks that it's better to see two old people, as he did recently, having a tremendous argument. "She would say something, and he'd say 'Cut the comedy' -- again and again. In the end they walked out arm in arm."

Hoffman divides his time between New York and Los Angeles. The children -- whom he tries to see as much as possible -- live with his wife in New York.

"You kid yourself if you think being separated does not have a traumatic effect on the children," he says. "They are going to feel that it is somehow expected that they favor one parent over the other, and that causes conflict. It is important for them to know that they can love both, that they can talk to Daddy about Mommy in positive terms and Mommy about Daddy in positive terms."

As for the adjustments he himself has had to make, he says in some ways it's not all that different, since his wife and children have always been based in New York, while he has had to travel.

As a parent, Hoffman sets high standards for himself. "I know that it's a fantasy, but I wish that I could be the kind of parent who is like the ideal director. That is, someone who guides you without your even knowing it, without imposing himself.

"I know that I have certain characteristics that have been rather constant through the years. For example, I always try as hard as I can at what I'm doing. While I'm not sure that it's right to impose that kind of attitude on my children, it's not wrong for them to know that's where I'm at right now. o

"The best thing I can say about being a parent is that you do the best you can and you fail a lot. I'm 42, I've been acting since I was 17, and I can still say I don't know how. All I know are certain things that I've learned.

"The biggest thing is that when it doesn't feel right there's a reason for it. So you go until it does feel right -- and no one makes you feel what's right more than your own gut. It's the same thing with being a parent. You learn from your mistakes. If you keep your eyes and ears open you can tell what doesn't work with your children . . .

"Maybe," he says, "it just comes down to making sure they know that you love them, that the house is not the same without them -- that they're special. Too often we say things like, 'You're just going through a stage.' But what makes life so interesting is that they're going through that stage and that they're there with you."

Hoffman is fascinated by the paradox of children. "They're their own, they're a mystery. You'll look at them and think that's my personality, my smile, my this, my that, but there's a nice bag of other elements there, too. And I enjoy not knowing. Besides, you never know how they're going to turn out.

"I know what I want, though. And I guess whether you like it or not you may influence them.

"I like them to like hugging, for instance, to be physical, to want to express themselves in that way.

"I like it when they talk about adults -- my friends or maybe parents of their friends. They'll say, 'That person's a phony.' I like that. You'll sit back and watch them really be able to pick up on things. It's innate. They smell fradulence early.

"I love it when Jenna says, 'Come here.'" He imitates how she motions him with her finger, holding it close to her face. "I say, 'What?' She says, 'Get down to my size.' I get down and she kisses me.

"I love it when Karina teaches me a bump and a grind, which is the latest dance. The first time it happened I got very shocked -- and I'm not easy to shock. I was embarrassed. It was a revelation . . ."

Hoffman doesn't necessarily think it's good that his children are growing up with material advantages. "I used to say to them, 'I want the day to come when you guys go to work -- when you spend the summer waiting tables to have the day on the beach and pay for the apartment you share with a bunch of other kids. I think that's a valuable experience.'

"Now I think it's better to say, 'When I did that I got a very special feeling that you can only get when you're earning your own money. There is a very particular feeling about that, which I think you should experience. It has its own kicker.'"

In "Kramer vs. Kramer," it would have been obvious to make Joanna Kramer -- the wife-mother who walks out on not only her husband, but her child -- the bad guy.

"We live in a society where a marital split-up usually means that the man leaves the house," says Hoffman. "Well, if there are children, in effect he's leaving the children, too. This is not only understood, but sanctioned. Yet, we think it's horrific if a woman leaves. Chastising her for that is one of the things this film challenges . . .

"When you think about what happens in many marriages, it's very painful. A husband and wife are supposed to be equal, yet it's not really that way. He's made the money usually, and there is the tendency to give him the edge. If they both work to buy a car, the tendency is to let him have the final choice because it's his money -- even though they both agree it's their money.

"A woman, whether she likes it or not, is going into a different kind of paternal structure when she goes into marriage. He's the breadwinner, she's the child -- that's the undercurrent. And whether the husband likes it or not, he tends to feel, 'I knocked myself out all day, why is this dinner cold?'

"I think a woman finds herself in a difficult position in that sense. But it isn't right, and it doesn't make anything work."

He thinks that men, too, lose in this situation.

He's reminded of a dinner conversation he once had with his wife, before their separation. Talking about one of the girls, she asked him if he'd ever noticed a particular birthmark.

"I said no," he recalls, "but that stuck with me, it bothered me. I remember looking at Anne and saying, 'I bet you know every mark on those kids -- every single mark.'

"There is something we miss as fathers," Hoffman says. "And I'm trying to change that."