Talk about a letdown. Sissy Spacek sounded so excited about this Thursday's ER, where she was slated to play the long-lost birth mother of Dr. Kerry Weaver (Laura Innes). At the last minute, she dropped out due to a "scheduling conflict" and Titanic's Frances Fisher was tapped to replace her. What exactly was Spacek's conflict?

"You know what? I don't feel comfortable talking about it," Innes politely tells TV Guide Online. "I know that it's a family matter with Sissy, and she's very disappointed not to be able to do the show. It's kind of her business. It had nothing to do with the show, it was more a family conflict that she had."

Fair enough. As for ER selecting Fisher to pinch-hit as Weaver's mommy, Innes says, "Frances was always among the people on the list. We felt like she's very right for the role. She's a wonderful actress and there is a sort of physical similarity between us. It's very different from other roles she's played. [Weaver's mom] is this very sweet, slightly awkward woman. It's a very tender little performance that she did."

Innes feels that exploring Weaver's abandonment issues will shed light on why she's been so, er, abrasive all these years. But it won't be easy — especially when the lesbian lady doc comes out to her birth mom, who happens to be a conservative Christian. That plot twist allows ER to tackle the very topical (and ever-sensitive) issue of gay tolerance in American families.

"Weaver's finally coming to some sort of healing or resolution [about being adopted] and this conflict emerges completely unexpectedly," Innes says. "Her mother is completely disapproving and unaccepting of her sexuality.

"After she overcomes her fear, Weaver becomes very articulate and passionate about defending her identity," she previews. "But her mother is also quite articulate — and [she's] not painted as a bad person or a stereotype. My feeling is that people are basically good. Once they come to understand and know each other, there's a lot more love, understanding and acceptance that can go on. It's very important that both voices be heard. As long as there's communication, it's a better world to be living in."