The star, who plays English slave abolitionist William Wilberforce in new film Amazing Grace, insists the role opened his eyes to the plight of those in captivity.
He says, "I was really rather ignorant of Wilberforce and the slave trade. But I fell in love all of it and strongly believe that slavery is still a very relevant topic today.
"We are more slave-ridden now than back then, and I think child slavery and sex slavery are the next two issues which need to be tackled."
Amazing Grace, premiered in London, March 19. General release is on March 23 across more than 200 screens.
In the US, Amazing Grace, which launched in February, has already grossed more than $10m at the box office, with more cinemas requesting to show the new film.
"About 100 new theatres have now asked to show the film and we hope to be able to meet that demand," said a spokesperson for Bristol Bay Productions, makers of the film.
"There have been truly exceptional attendances in cities like Los Angeles, Seattle, Dallas, Las Vegas, San Francisco and Washington DC."
William Wilberforce was elected to the House of Commons as MP for Hull when only 21 and dedicated his life to the reformation of manners in order to build a just and fair society.
He advocated prison reform, better hospital care, improving conditions for the poor and other areas of social reform, but his passion was to abolish slavery in all its forms. In 1807 the Commons voted to abolish the slave trade throughout the British Empire, but it was not until 1833 that total abolition was achieved.
Three days after this latter event, in July 1833, Wilberforce died. The hymn 'Amazing Grace' from which the movie derives its title, stands as the personal testimony of John Newton, a former slave trader who had a dramatic conversion to Christianity.
Wilberforce and Newton met on several occasions and Wilberforce used the words of Newton's hymn to prick the conscience of influential members of the Commons and the Lords to support the Anti-Slavery Bill. John Newton died the same year as the bill passed into law in 1807.
Numerous events are continuing across the UK to celebrate the 200th year since the abolition of the slave trade across the British Empire. Shadow Foreign Secretary William Hague and Alan Johnson, the Secretary of State for Education and Skills, have invited fellow MPs to a special parliamentary screening of the film.
