Edge of Darkness was supposed to be a small thriller that would let Mel Gibson ease his way back into acting roles -- his last movie was Signs, which was followed a few years later by his drunk driving arrest -- by starring opposite another big star: Robert De Niro.

That's still the plan, but he'll now have to make a go of it without De Niro. The actor -- who stars in next week's Righteous Kill -- walked off the set yesterday after only a few days of filming. Production is continuing; they've re-arranged the schedule to shoot non-De Niro scenes until they can find a replacement for him.

It's too bad, because the project sounds like something that could be pretty special. The director is Martin Campbell (Casino Royale), working off a screenplay co-written by The Departed's William Monahan. The story follows a homocide detective for the Boston Police Dept. who's investigating the murder of his own daughter. He uncovers a cover-up involving corporations and the government; De Niro's character was an operative sent to cover it back up.

Expect an announcement in the next few days about De Niro's replacement; names like Tommy Lee Jones and Morgan Freeman have floated around, but at this point it's all rumor.

De Niro's reason for leaving so abruptly, by the way, was "creative differences," an oft-used blanket term for some kind of behind-the-scenes fight. Again, expect details to leak out, hopefully sooner rather than later. ❏