The 43-year-old actress, who played Narcissa Malfoy in Harry Potter, was speaking to ITN on the red carpet for the premiere of hew new film Hugo.
She explained: "So often when you meet child actors they're weird, they're freaks. No, I mean it, they're really odd people.
"Because they have a very weird life that as an adult you can just about get your head around."
McCrory praised 14-year-old fellow Hugo cast member Asa Butterfield, who plays the eponymous character Hugo Cabret in the movie, for being one of the exceptions to her rule.
"For a child to go through that and not end up very strange is really exceptional, and he's managed it," she added. "He's a very bright boy which comes across on screen but has served him well in life."
> Helen McCrory: 'Daniel Radcliffe is Harry Potter ambassador'
Watch Helen McCrory and Asa Butterfield in the trailer for Hugo below: