Bernice (LisaGay Hamilton) and Fontayne (Yolonda Ross), the title characters of John Sayles’ noirish thriller “Go for Sisters,” are not biological siblings but former high-school friends who once looked so much alike that people said they could “go for sisters.” In their tense reunion 20 years later, in a seedy Los Angeles outlier, they are on opposite sides of the law.

Now a parole officer, Bernice is assigned to the case of Fontayne, a recovering drug addict newly released from prison. The movie’s opening scene, in which Bernice turns a deaf ear to the pleas of a parole violator, reveals her to be a stern, dispassionate woman who has heard it all. She is not so much hardhearted as levelheaded. Taking pity on Fontayne, arrested on minor parole violations, Bernice gives her a second chance, and the friendship — severed when Fontayne stole away Bernice’s boyfriend — is renewed.

Bernice’s estranged son, Rodney (McKinley Belcher III), has been involved in human trafficking across the Mexican border. When he goes missing after the murder of a partner, he becomes a suspect. In return for letting Fontayne off, Bernice enlists her to use her underworld connections to help her find her son.

Their deepening friendship is the heart and soul of “Go for Sisters.”

The creation of layered characters you care about is what Sayles does best, and “Go for Sisters” includes three. The third, Freddy Suárez (Edward James Olmos), is a disgraced former Los Angeles police detective whom Bernice hires to guide them through Tijuana, Mexico. Olmos has never looked craggier and more weathered portraying this tough gumshoe.

Surveying Tijuana, he growls the movie’s best line: “This isn’t Mexico. This is like a theme park for bad behavior.”

As long as “Go for Sisters” is focused on its characters, it remains on firm ground. But the flimsy detective story draped over them is underdeveloped and too sluggishly paced to take hold. This self-financed movie badly needs a dash of Hollywood-style action.