The Downton Abbey creator objected to the inaccurate and "unfair" portrayal of first officer William Murdoch, who in the 1997 Oscar-winner is seen committing suicide after shooting dead two passengers amid the sinking.
His surviving relatives received an apology from producers about his treatment, having complained that the real Murdoch worked diligently until the ship's final moments and perished at sea.
"That was very unfair how Murdoch was depicted. He wasn't cowardly," Fellowes told the Radio Times. "He fired the pistol to just stop a potential riot. It was suddenly getting out of hand, and he fired it in the air. That's not being cowardly."
Claiming that Murdoch is credited with saving the lives of 75 Titanic passengers, Fellowes continued: "I don't think you can just say, 'Well, we'll make this guy a villain - he'll do'. I think with real people you have a kind of imperative to be true to who they were.
"I don't think you can take someone who was moral and decent and make them do something immoral and indecent. I would feel uncomfortable doing that. So we do have Murdoch, and we have him firing a pistol, [but] there is a little bit of setting the record straight."
Fellowes previously said that his version of Titanic will tell "a very different version" of the ship's sinking.
The four-part series will premiere Sunday (March 25) at 9pm on ITV in the UK. It will be shown on ABC in the US.
Cameron's film will be re-released in 3D on April 6 to mark its 15th anniversary and the centenary of the sinking.
> 'Titanic': What's Julian Fellowes's new drama like? - Preview
Watch the trailer for Julian Fellowes's Titanic below: