What the hell’s going on with Sidney Lumet? Last year the high-class director of ”The Verdict” and ”Dog Day Afternoon” went slumming with the silly ”A Stranger Among Us”; now he leaps into the gutter with screenwriter Larry Cohen, the low-rent brain behind schlockers like ”It’s Alive.” Cohen wrote this entertainingly stupid trashfest, in which a high-priced lady lawyer (De Mornay) defends a slick ladies’ man (Johnson) who may or may not have offed his wife. Yes, the movie’s a pile of ”Jagged Edge” leftovers (down to Jack Warden in the Robert Loggia crusty-detective role), but they’ve been overheated to a tasty absurdity — and Johnson has a high old time as an oily gigolo mesmerized by his own charm. The dialogue reeks, the plot takes so many twists it bumps into itself coming the other way, and neither De Mornay nor Lumet seems to be in on the joke. In other words, a bad-movie fan’s delight.