Sonny Steelgrave was a hard act to follow.

The mob kingpin had been a staple in the first nine weeks of "Wiseguy." That's the new CBS series gaining favor among the small but hardy band of viewers who appreciate its fine writing, acting and tension.

Talk about tension: The show's hero, deep-undercover agent Vinnie Terranova, is never more than a step away from a rubout -- hey, he took two sniper's bullets in the chest a couple episodes back.

But Steelgrave. He was played by Ray Sharkey. Everybody loved him. Or hated him. Or both. And just as people were loving to hate him or hating to love him, he was gone. And on came Mel Profitt, played by Kevin Spacey. Who is this guy?

"It's a risk {for the show} to end the run of a bad guy who became popular," said Spacey. "But it makes the character of Vinnie more credible. If he went 62 episodes without getting anyone arrested, he'd be a bad undercover guy."

That was the situation Spacey parachuted into, becoming Mel Profitt, the show's new male heavy. He's not just mean: He's twisted. And he's accompanied by Joan Severance. She plays his sister Susan. Sure she's his sister. She's twisted too.

"The crew loved Ray Sharkey," said Spacey. "Then, here was this New York actor dropping in from the sky." But the crew warmed to Spacey and the audience warmed to Mel. And now it's all over. Mel Profitt takes his last twist Monday night.

But Spacey. Who is this guy? Or after Monday, who was that guy?

Yes, he blew in from New York, his arm twisted behind his back by his agent, to check out the L.A. scene when new series were being cast. Next thing he knew, he was talking to Stephen Cannell, the ubiquitous television producer and "Wiseguy's" creator, and heading for Cannell's Vancouver branch office.

But he wasn't a stranger to the West Coast. Grew up there. And he's not a newcomer to acting, even though he is a fresh face. He's been doing it since he was 14. For him, that's half a lifetime ago.

He does have something of a New York stage actor's attitude toward television or, better, a point of view. "In a lot of television, there's a mentality: We don't want it good, we want it by Thursday. I wanted to bring a level of mystery to the part of Profitt so people would want to watch and to give the part a level of humor," said Spacey. "A slightly more bizarre sense of abandon is how I approached it."

It must have worked. People stop Spacey on the street. They call him Mel. They quote dialog from the last show.

"People seem to respond to the humor," he said. "I had reservations about doing that kind of a thing. The first script was interesting. It showed promise of opening up the character. But I didn't know where it was going."

Promise and possibilities. "I got the second and third scripts during the filming of the first," he recalled. "These had a quality I had not seen Mel have -- a childlike playfulness. The third one informed me so clearly how to play a scene in the first script -- his playfulness, his childlike sick humor. So when the audience gets to the third episode, that quality had already been hinted at."

So you're not surprised when he wants to buy a major league team so he can play second base. And then turns vicious when he's turned down.

You might be surprised to see Spacey turn up in such a featured role. His is not a common TV face. He started out in Los Angeles, where he acted in plays in junior high school. When he got to Chatsworth High School, his classmates included Mare Winningham and Val Kilmer.

Kilmer went on to New York's Juilliard School. Spacey went on to sell shoes and do construction work.

Kilmer kept in touch and encouraged Spacey to come east. In 1979, Spacey auditioned and was accepted at Juilliard. More successful classmates: Kelly McGillis, Elizabeth McGovern. Then it was on to small parts in Shakespeare in the Park productions. Producer Joseph Papp noticed, hired him as a production assistant. Then Spacey landed a part in an off-Broadway show, "Barbarians."

"Joe Papp saw it and fired me," said Spacey. "He was afraid I might get overly comfortable with production work. It was the kindest gesture, a very fatherly thing for him to do. Four months later, he was in the audience when I appeared in 'Ghosts' on Broadway."

But after "Ghosts," Spacey sat down for a heart-to-heart with his agent. "I wasn't good enough," concluded Spacey. So he and Slaight hit the road for two years of regional theater.

Slaight. That's Spacey's dog. Wherever you find Spacey, you find Slaight. Spacey found Slaight, a stray Labrador, in 1981. Slaight travels in his own cage, hangs out on sets.

But back to Spacey. "In late 1984 I was back in New York, with more work under my belt -- some good, some not so good." Good enough to get parts in "Hurlyburly" and the film "Heartburn," both directed by Mike Nichols.

By 1986 he was in "Long Day's Journey Into Night" with Jack Lemmon. Producer George Stevens Jr. saw the play, and when he cast the TV miniseries "The Murder of Mary Phagan," he brought Spacey along with Lemmon. Spacey played the newspaper reporter. Later this year, Spacey will be in a theatrical movie, "Rocket Gibraltar," with Burt Lancaster.

Meantime, he had to be dragged to L.A. for the pilot season. "I was just going out to meet people and say hi," said Spacey. "There's nothing that depresses me more than knocking on doors in L.A." He didn't have to. From the airport he went to Cannell's office. Three days later he was in Vancouver. "It was," he said, "the most spontaneous decision I ever made."