Viola Davis has branded the idea of Julia Roberts being cast as Harriet Tubman ''ridiculous''.

Screenwriter Gregory Allen Howard recently revealed he first suggested a movie about the African-American political activist over 20 years ago and was shocked when executives suggested the white 'Pretty Woman' star would be perfect for the role.

And now Julia's 'Eat Pray Love' co-star has not only slammed the idea, she claimed it happens ''all of the time''.

Viola, 54, said: ''It happens all of the time [in Hollywood].

''Here's the thing, simply put: Julia Roberts as Harriet Tubman is ridiculous. That barely warrants a response. That's ridiculous.

''I understand that the film industry very much is about commerce and money, I get it. But that's ridiculous.''

The 'How To Get Away With Murder' star admitted she worries that the issues of diversity and inclusion are never raised to the people who should be questioned the most.

She added to MadameNoire: ''I'm always a little concerned that the people who are questioned about race and diversity and inclusion, are the people in need and not the people in power.

''You don't question the people who have not been invited to the party, you question the people who are throwing the party.

''And the people who are out there who are living in any way, even within this industry, who have the green light vote, who have the power to finance films, who are making those choices to want to cast a Julia Roberts as a Harriet Tubman, this is a better question for them.

''Why is it that you are not armed with enough information and why you don't see it as important to cast a Black woman as Harriet Tubman? Why is it that you would want to make this decision? You can go through the history books and see that Harriet Tubman was a Black woman.''

Cynthia Erivo has taken the lead role in biopic 'Harriet'.