THE REHEARSAL room is a converted schoolroom. The walls are brightly painted, the floor linoleum, the scale of everything is just a little tinier than usual. Outside the large windows is a glorious autumn afternoon. Inside is a palpable anticipation among the cast of the Round House Theatre's production of "The Butterfingers Angel," which opens tonight. On this particular Sunday they have a special visitor -- William Gibson, the author of the play.

Gibson is taking a break from New York rehearsals of his newest play, "Morning after the Miracle," starring Ellen Burstyn, to answer questions, watch the scene work and hear the score.

The cast gathers around the angular 67-year-old, whose eyebrows are like two silver and black caterpillars having a meeting in the middle of his forehead. He describes how this surreal telling of the Nativity story, inspired by "The Cherry Tree Carol," came to be written.

Gibson, who lives in Stockbridge, Mass., "fell in love" with a Jesuit chapel in nearby Lenox. So he wrote a passion play for Easter to be performed there. It was a success, so he wrote "The Butterfingers Angel" for Christmas, with traditional carols untraditionally arranged. But the Jesuits wouldn't allow it to be done. They were afraid that its unconventionality would have people saying " 'There go the Jesuits again.' I'm not interested in psychologizing away the virgin birth," he tells the company. "I'm interested in the tensions the situation creates in real people."

One of the children in the cast has a basic query: "What is the main theme?" She receives a basic answer from Gibson: "Life is the only value we have." Someone comments that Mary, as written in the play, is a feisty lady. "Mary comes out of a tradition of women I write about. My wife, mother and grandmother were all strong," he says. "Pains in the neck."

Director Jerry Whiddon and Gibson slide into a private conversation as the cast gathers around the electric piano to begin doing some vocal warm-ups. The room is filled with the sound of banshees going budda-budda-budda-budda and slapping their thighs. When they're loosened up they start singing music director Chris Patton's renditions of such favorites as "Joy to the World."

Patton, "a Congregationalist from New England," says arranging carols is something he has wanted to do. "Every composer wants to get his hands on them. 'Adeste Fideles' in Latin is a stitch to do as a jazz tune."

Then it is time to work through "the slaughter of the innocents." The rehearsal explodes. The cast is pushing hard, trying to impress the playwright. But it is far too soon in the rehearsal process. Lines and stage blocking get lost. Whiddon trys to cool his eager company down. "I know it's tempting to get caught up in the energy," he tells them. "That'll come."

Every one takes a long five, and Death is the topic all over the room. Gibson and Whiddon are discussing how to most economically stage the slaughter scene, avoiding guignol and yet conveying the horror. Over by the piano actress Kathy Silvia is showing colleague Sharon Burke how to fall dead without killing herself.

Someone asks if it's fun to die on stage. Silvia, a graduate of the University of Maryland, thinks it is. She gets wide-eyed, thinking of the intensity she reached in a play called "Dark of the Moon." "First the baby dies, then you get to die," she laughs. "It was the perfect role." Silvia is familiar with Gibson's plays. She worked on Gittel Mosca, the heroine in "Two for the Seesaw," while in college. She is a little intimidated by Gibson's presence: she says she would like to say something to him but doesn't quite know what.

Director Whiddon calls everyone together and they work a few more scenes for Gibson. But people are slowing down as the afternoon approaches the border of the evening, and it is time to end the rehearsal.

Gibson kisses Lambert and hugs Michael Littman, who plays Joseph. Silvia slides unobtrusively towards them. Hoarsely she says, "Mr. Gibson." At just the same moment someone else calls to him and he turns away. She starts to put on her coat, seemingly embarrassed and a little hurt. Two minutes later he seeks her out. They talk, a small, polite talk about Gittel.

The evening has arrived and Gibson is rushing back north. He calls to the group: "I'm going to try and see you -- when you know your lines."