The actor â an ambassador for charity Unicef â met with the royal couple in Copenhagen, Denmark, where they were helping aid relief efforts for famine stricken parts of Africa by packing boxes.
He told the Daily Mirror newspaper. "They are both very interested in east Africa, it's so close to their hearts.
"It was just reiterating their passion for this cause and I think it must be a passion otherwise the royals wouldn't just give up a day of their very busy lives and the high demands there are on them for attending all sorts of functions and that's apart from the Raf duties of the duke."
The famine affected east African region is especially close to the newlywed royal couple's hearts, after William proposed to Catherine while they stayed in a log cabin in a remote part of Kenya in 2010.
Roger also said the royal's efforts in the campaign â where severe food shortages are affecting some 13 million people â echo those of William's late mother, Princess Diana.
He added: "I think comparisons are odious but [Diana] did wonderful things in bringing the world's attention to the danger of landmines and I think that their royal highnesses are showing an equal interest."
The Prince and Duchess were joined in their work at the emergency supply warehouse - a worldwide distribution centre for the charity â by Denmark's Crown Prince Frederik and Crown Princess Mary.