WHETHER he’s playing an American patriot, a suicidal cop or even a man without a face, Mel Gibson has always been sexy onscreen – until now.

With the addition of a pair of Coke-bottle glasses, a balding pate and a doctor’s white smock, Gibson is unrecognizable as a 65-year-old oddball psychiatrist in “The Singing Detective.”

“You think of Mel as this sexy, vibrant guy, and here he is playing a nerdy, wizened little man who reminds me of nothing so much as Henry Fonda in ‘On Golden Pond,’ ” says director Keith Gordon.

“He’s found a voice that doesn’t sound like Mel Gibson and he moves much slower than he normally does. A lot of people haven’t even recognized him until the third scene he’s in.”

Gibson – who’s also a producer on “Detective” – is nothing if not a good sport: He even takes part in an hilarious duet with co-star Robert Downey Jr., lip-synching to Eddie Cochran’s “Three Steps to Heaven.”