NEW YORK -- Boston writer Dennis Lehane had this opening sentence kicking around in his head for eight years, but no place to put it: ''Your father picks you up from prison in a stolen Dodge Neon, with an 8-ball of coke in the glove compartment and a hooker named Mandy in the back seat."
He couldn't for the life of him find the right vehicle for it; it just didn't fit into anything he was working on. Then he was asked to write a short story for an anthology, with one requirement: It had to have something to do with fathers and sons.
''Until Gwen" was also published in The Atlantic Monthly and was selected for both ''The Best American Short Stories" and ''The Best Mystery Short Stories" of 2005. Though the story has to do with a father and son, as it turns out, brothers figure prominently in it, too -- specifically, Dennis and Gerry Lehane.
Dennis, whose best-known work is ''Mystic River," has turned the story into a play starring his big brother Gerry. ''Coronado" opens in New York tomorrow for a three-week run at the Manhattan Theatre Source in Greenwich Village. It's a complex story of intertwined lives, of the thin line between love and hate, between sanity and madness.
Last week Lehane flew in from St. Petersburg, Fla., where he is a writer-in-residence this year at his alma mater, Eckerd College. It was eight days before show time, and the actors in this off-off-Broadway production were nervous. What would the playwright think?
In ''Coronado," Gerry plays the role of a violent, sociopathic father -- the opposite from their own dad, Michael. ''He's the sweetest guy in the world. He worked his whole life in Sears and Roebuck," said Dennis.
Gerry and Dennis are the youngest of five siblings; the next one, another brother, is six years older than Gerry. They grew up near Edward Everett Square in Dorchester and attended Boston College High School. Ironically it was Gerry who had the quiet introspection of a writer, while Dennis had the more gregarious personality of an actor.
''But he always wanted to act, and I always wanted to write, and I've long suspected that at some point we agreed on this unspoken contract where I'd never try his thing and he'd never try mine," said Dennis. The brothers grew up close. ''We always had each other's back," said Dennis, who credits the theme of loyalty, which runs through much of his work, to their relationship.
''Gerry always kind of looked after Dennis when they were kids, and I'm very proud of both of them," said their mother, Ann, who lives in Plymouth. ''Their success is great, but to me they're wonderful kids, they're good, they're caring."
Ann and Michael Lehane are headed to Florida for the winter, and will likely miss the play. Which is a relief to Dennis. ''It's very important my mother not see this play," he said, laughing.
Ann Lehane has long told her youngest that she loves his writing, but disapproves of the profanities. ''He writes great stuff, but he should leave the swears out of it," she said. ''I don't know where he learned to swear."
In a pep talk before the first scene, he told the cast of nine to ''find the comedy" in the play, which is vintage Lehane: gothic, with patches of perverse humor. ''There's so much darkness in this world, we've gotta find the fun or otherwise the audience is going to be depressed," he said. Indeed, during the first scene, he wrote on his pad: ''Everyone is playing too loud, too angry."
During a break, Dennis told the cast to cool it with the overwrought Southern accents: ''It's making them go into caricatures rather than characters. We don't want to come across as a bunch of East Coast elitists making fun of the South. I've spent too much time in the South, and I love it." He told one woman to make a closing line stronger: ''Women have a way of delivering really killer lines as they walk away." And he told a guy that he didn't act ''nearly drunk enough" in another scene. At the end of the rehearsal he hugged a young actor and announced that he loved the entire performance.
It is impossible to view a loved one objectively -- especially one you shared a bedroom with growing up. Both brothers said it was only recently that they could judge each other's work with a professional eye. In separate interviews, each used the same word -- ''shocked" -- to describe their reactions to that work.
''Seeing my brother acting tonight, I was shocked," said Dennis. ''He was great. He went places [with a costar] I didn't know they were going. They played it far more emotionally than I wrote it."
Gerry said he has the same reaction to Dennis's writing. ''Sometimes, I'm still shocked at how good he is. I've had to take it on faith from other people who tell me he's terrific. He's my brother; there's no suspension of disbelief that's necessary to make a judgment."
In fact, Dennis said he has been unable to see Gerry's work clearly until that night's rehearsal. ''Seeing him, I thought, 'Oh, that guy's good.' Then, 'Oh, it's Gerry!' "
''It's been a blast working together," said Dennis.
''Being in two creative fields . . . in the back of my mind, I think I hoped it would happen," said Gerry. ''I think he has an ear for the truth." He turned to his brother. ''Even in your very first book there was a sense of this in how people would behave and speak."
''Thank you," said Dennis, sipping a beer.
OK, he's a good writer. But what kind of brother is he?
''He's a great brother," said Gerry.
Dennis laughed. ''And I have a pool table."
''And he lets me stay free," Gerry added.
In fact, it was during one of those free stays, last Christmas, that the play was hatched. Gerry had come to Boston to spend the holiday at Dennis's house, and had brought a couple of actor friends. Dennis gave Gerry a copy of his story to read. Gerry and his friends were to visit for only a couple of days. But once they started talking about the play, they stayed through New Year's Day.
''This idea started percolating, but we couldn't figure out how to put it on the stage," said Dennis, whose other novels include ''Shutter Island" and ''Gone, Baby, Gone." This is his first play. ''It had this whole feeling of, 'My Dad's got a barn, let's put on a show,' " he said.
Dennis, who is working on a novel about the Boston Police strike of 1919, said he would love to bring ''Coronado" to Boston. ''I'm waiting for the reviews, to see how much rewriting I have to do. I still consider it a work in progress. It's got way too many references to God in it." The play will have a one-night stand in St. Petersburg in January, at the end of a writers' conference Lehane is organizing, and will open there in March.
People have questioned why he has done the play; they've told him there's no money to be made. But the way he sees it, there's no pressure, either. ''If we go out and fall on our faces, it's OK. I never expected to be successful. I never cared about 'stuff.' Now I have the freedom to do what I want. If I want to write a play and make no money on it, it's OK. This was my dream, and I'm living it."