Murray, 49, who has carved out a comfortable television career without infringing on the considerable profile of his older brother, comic legend Bill Murray, figured he was probably good for ''guy in office'' before Goldthwait shocked him with an unexpected offer.

''I told Bob that it was dark and twisted and that I really liked it, and then he said I'd be perfect for the main part,'' Murray says. ''I have to admit I was surprised because it's very rare that I'm offered a lead role.''

Murray is best known as Freddy Rumsen of Mad Men's fictional 1960s advertising agency, whose workplace drinking ended in public humiliation and dismissal before he cleaned himself up and was eventually rehired. Despite his failings, Rumsen is genuinely likeable, and that same quality has helped keep Murray employed in a succession of Hollywood television series, including Dharma & Greg, Still Standing and Shameless, over the past two decades.

God Bless America makes use of that affability but turns it into a trap for the audience's sympathy. A middle-aged, divorced father, Murray's Frank is a decent man slowly going mad in an America where the toxic chatter of reality television and celebrity gossip has disfigured everyday discourse, made offensiveness the norm and produced successive generations of the selfish and self-entitled.

''Why have a civilisation any more if we're not interested in being civilised?'' wonders Frank. And once he's lost his job and been told he has a terminal illness on the same day, he decides he may as well spend his final weeks satisfying the murderous revenge fantasies that repeatedly occur to him.

Accompanied by a caustic teenager, Tara Lynne Barr's Roxy, the ''platonic spree killers'' brutally gun down right-wing cable pundits, teenage reality show stars and even obnoxious people who talk during a cinema screening.

''You have to deal with your own beliefs when you find yourself laughing at some of these things,'' Murray says. ''You think you can justify what these characters do, but in some cases they're killing people who've just had a bad day and let it affect them. It's actually not OK, but once the film gets on a roll, it pulls you along.''

Murray calls it a ''very violent film about the need for kindness''. But if the initial fear was that this low-budget mix of murder and manners from Goldthwait - the jittery 1980s comic of Police Academy fame turned industrious independent filmmaker - would offend audiences, the reality has been the opposite.

''Most people laugh at the movie, they get it, but the complaint we do receive is that we didn't go far enough,'' Murray says. ''Some people just wanted 90 minutes of obnoxious jerks getting murdered. No characters, nothing about the relationship between Frank and Roxy, just pure vigilante shootings.''

Murray, who is the product of a large ''Chicago Irish-Catholic family'' and looks it, has a dry, quiet sense of humour, but without the bemused, deadpan qualities that are his older brother's trademark.

He was still in high school, as the captain of the football team, when Bill Murray shot to prominence in the late 1970s. And although Joel initially turned down a small role in the anarchic Caddyshack in the 1980s, he followed several of his brothers into showbusiness, finding his niche as a supporting player.

In God Bless America he was not only in virtually every scene, juggling bleak comedy and dramatic monologues, but on the democratic low-budget set staffed by friends he helped move gear and suggested scenes.

''Normally I'm the guy who jokes around on set, does his job when required and then heads home for the day,'' he says. ''This time I was doing one demanding scene after another and whenever I wasn't working I was running my lines with anyone who could help. After 25 years, it appears I've graduated to independent films.''

God Bless America screens from December 6 at Dendy Newtown.