However, he thinks it's "inevitable" that the royals won't like Spencer.
He told Us Weekly: "The royals won't like [the biopic] at all."
Robert also observed that Kristen, 30, will be under a huge amount of pressure playing the iconic royal.
However, he conceded that the actress looked remarkably similar to the late princess in a recent publicity photo.
He reflected: "I think the most important thing is how well the actress can interpret the role. We'll have to wait and see on that.
"As for the photographs of her, I thought she looked remarkably like the photograph of [Diana]. Having been someone who has met Diana and knew her fairly well, I thought it was uncanny, really.
"But we'll just have to see. I think what's more important is the authenticity of the portrayal, and we'll have to see how that develops because my understanding is it's going to be a good script and it's going to be a good premise, but it might not necessarily have actually happened in the way they're saying."