The Harry Potter actress left the hip-hop star speechless within minutes of beginning their interview on The Graham Norton Show when she admitted she was transfixed by him.
Margolyes told him, "I'm just fascinated by you because unfortunately, I don't know many black people, except in showbusiness - and that's what's so nice... I don't know much about rappers, you're the first one I've actually talked to."
The audience gasped but the Black Eyed Peas star appeared to take her comments in good humour, telling her, "I'm the first rapper and black dude you kicked it with," before teaching her the meaning of the slang words 'homeboys' and 'old school'.
Margolyes later heaped praise on will for his charity work, insisting, "You're fabulous, you really are... You're making a difference, and that's what we all want to do in our lives. How unexpected that this rapper would do this!"
The episode aired in the U.K. on Friday (22Jun12).