MEAN STREETS

Robert De Niro--famed for going to lengths (and weights) for his art--spent the past three weekends in Chicago hanging out with cops as he readies for a role as a cop-turned-bounty hunter in Martin Brest’s “Midnight Run,” an action-comedy shooting later this year for Universal.

“But you wouldn’t know it’s De Niro,” said a source who met the star when he was disguised in “prop glasses” and ultra-conservative suits.

The story, as scripted by George Gallo, finds De Niro eluding both the mob and the FBI--handcuffed to Mob accountant Charles Grodin.

During De Niro’s research, said our source: “He accompanied this one cop on a shakedown of a guy who had killed three people. Then, De Niro asked to see the bodies.”

The actor’s reaction to real-life corpses? “He excused himself--and headed for the men’s room.”