It's hard to imagine "The Sopranos" without Livia, the malevolent mother played by the late four-time Emmy winner Nancy Marchand, who died of cancer Sunday at her home in Connecticut. If James Gandolfini's Tony was the dramatic muscle of the show, and Edie Falco was the heart, Marchand was the guts -- maybe the brains, too.

Her character was as smart as anybody -- a real schemer, like the same-named bad mama from "I, Claudius" -- and she made herself more deadly by appearing frail. Or perhaps she really was frail � her body weak, her memory and emotions scrambled by Alzheimer's and a stroke � yet so full of rage and bitterness that she simply couldn't stop herself from doing horrible things. My late grandmother would have called her a real piece of work.

It's tough for a show to go on following the death of one of its most important players. Even the most crackling ensembles are staggered by real-life tragedy.

"Chico and the Man" and "Cover Up" were essentially destroyed after the deaths of their lead actors (Freddie Prinze and Jon-Erik Hexum, respectively). "NewsRadio" was severely damaged after the passing of the great Phil Hartman and limped to a close a year later. It took a couple of seasons for "Cheers" to overcome the death of Nicholas Colasanto, who played Coach; Woody Harrelson worked as a replacement partly because his dopey delivery suggested a younger version of the character. "Hill Street Blues" was hit hard by the death of Michael Conrad, the gentle-spirited sergeant who delivered the signature phrase, "Hey, let's be careful out there."

Yet all the aforementioned actors, however charming and talented, played much less complex and demanding roles than Marchand played on "The Sopranos."Creator David Chase is a dedicated and resourceful storyteller. He might find a way to paper over the gaping narrative hole created by Marchand's passing. But it's the highest praise to Chase and his writers to say that it will be difficult -- maybe impossible.

The best characters on "The Sopranos" have a couple of layers. Livia had six or seven, and the way Marchand played her, you could never be sure which ones were real and which were invented. She was in her late 60s when she donned Livia's bathrobe and slippers, but her performance was more exciting than any Method rantings by stubbly-cheeked twentysomething brooders. Her Livia moved like a tortoise, but if you think that was a handicap, ask the hares. The character's death deserved a three- or four-episode arc; now it will have to occur off screen, and the awkward real-life reasons for this approach will demand that loose ends be tied up in an hour or two.

Last January, during a press conference for the series, Chase told reporters he hadn't really planned what to do if Marchand became too ill to work (or worse). He said "To think ahead would be just too strange." Chase repeated that sentiment in comments published by The Star-Ledger yesterday, saying he and Marchand had discussed what to do in the event of incapacitation or death, but had mutually decided to just keep going and not think about it.

Uncharitable viewers could point out that such an attitude, while philosophically defensible, did not serve the show. We saw very little of Livia this season -- at least compared to the first season, which she ruled like Don Corleone, making her presence felt whether she was actually in a scene or not. In retrospect, it would have been nice to have seen a bit more of Livia's prickly relationship with her blood family and a bit less of the scary-zany supporting characters played by David Proval and Aida Turturro, who were developed at length, then eliminated from the drama by, respectively, gunshots and a bus ticket.

But under the circumstances, Chase and Marchand's response to her cancer seems both appropriate and humane. Marchand knew she wasn't in good health when Chase hired her, and she told Chase. Chase gave her the part anyway, and rather than writing her out of the show when she became too sick to work regularly, he ratcheted back her screen time and permitted her to participate to the extent her body would let her.

Show business is not known for such generosity. Whatever difficulties Marchand's death causes "The Sopranos" creatively, its cast and crew should take pride in the fact that they bent over backwards to help a great actress keep working.