Robbie Coltrane's brilliant performances stand as legacy to the British import "Cracker," which airs its series finale Tuesday night on the A & E cable channel. Yet Coltrane's full-throttle bravado works best when reflected off the quieter, subtler work of co-star Geraldine Somerville. As the independent Detective Sgt. Jane Penhaligon, Somerville projects both the strength and vulnerability of a character who has survived rape by a colleague, his consequent atoning suicide, career frustration and an ill-fated affair with Coltrane's Eddie "Fitz" Fitzgerald. In this last episode, about a female serial killer with an obsession for Fitz, Penhaligon is subjected to yet more embarrassment before her co-workers when the suspect reveals during interrogation that the detective had been sleeping with Fitz. Tuesday's "True Romance" turns the tables on consulting police psychologist Fitzgerald when the killer, a dropout psychology student, does her own analysis of the gambling, heavy-drinking, married father of three. Somerville, in a phone interview from London, explained Penhaligon's attraction to the dysfunctional Fitz: "She plays everything by the rules and this man comes along who understands her immediately. Secondly, he doesn't play by the book - he breaks all the rules and to her I think that's mentally attractive because she's so confined by the rules." 

Cracker has broken rules also, providing a compelling drama that looks not just at brutal crimes but also into the minds and souls of the criminals. That approach, Somerville said, has made an impact in England. "It's been amazingly popular and quite controversial as well." Somerville credits series creator Jimmy McGovern for much of the show's success. McGovern, she said, came along at a time when most TV dramas there were "easy-watching" and politically correct. "He's basically saying, `Let's look at who these people are, let's get emotionally involved with somebody who does something so heinously awful, understand or try and understand where they're coming from.' And you do get emotionally involved with these people when you're watching it on the telly. You do actually connect and think, `I understand that feeling they have, I may not go out and kill someone, but I understand it.' " 

Such empathy initially may have made viewers uneasy, but for a 27-year-old actress the dichotomy provides a wonderful opportunity to ply her craft. The three-year stint on the psychological drama has been Somerville's biggest career break. Born in Ireland and raised on the Isle of Mann, the actress, who first wanted to be a ballerina, trained at a drama school in London before starting her career, which has included theater and television guest roles before being cast as Coltrane's star-crossed love. Somerville credits the success of the collaboration and the popularity of the characters to the positive working relationship she shares with Coltrane. "With an ongoing series, it's so much about the chemistry between actors. That's what informs everything. It's wonderful to work with an actor when you have chemistry with him. And I don't mean necessarily a sexual chemistry - I just mean a chemistry, you work well together. And I certainly feel that Robbie and I had that . . . In fact, most of the actors had that on Cracker".