Keaton gets her chance to chew up the scenery

It’s the rare actor who can pass up the opportunity to play a character grappling with substance-abuse issues, and there are at least a couple of reasons why. One is the public-service aspect, which is what Oscar winner Diane Keaton cited as motivation for taking the role of a cash-strapped, drug-addled mother of two in tonight’s Lifetime movie “On Thin Ice” (9 p.m.).

Keaton, also an executive producer on the project, said she wanted to show “what can happen when you are living in America without money and don’t have any options.” But another reason actors find such parts irresistible is the chance to really let it rip and careen full tilt around the ecstasy-and-agony arc. The results can be enormously powerful, painfully overwrought or somewhere in between, which is where Keaton’s handling of the role lands.

Keaton stars as Patsy McCartle, who since her husband’s death five years earlier has been struggling to keep food on the table for herself, teen son Jason (Michael Seater) and a younger boy (Colin Roberts as Kevin) whose health problems further strain the finances of a household whose wage-earner juggles jobs as a janitor and a waitress at a local diner.

Yet frazzled Patsy seems to be a decent mother, with Keaton using her familiar laugh and endearing eccentricities to good effect early. But when Patsy gets a layoff notice from the diner, the house of cards comes tumbling down. A crystal meth-dealing neighbor persuades her to sign on as a courier and Patsy spirals downward. Keaton’s chain-smoking, Medusa-coifed take on Patsy’s drug addiction isn’t exactly over the top, but she’s in the neighborhood. Her meth-kicking histrionics are particularly tough to watch, and not necessarily for the right reasons.

But the McCartles’ plight offers suspense down the stretch, with the film’s public-service mission largely accomplished.