Royo had been working on screen and in theatre productions as an actor for years, and took his role very seriously - but inhabited the character of drug addict Reginald "Bubbles" Cousins so well that he was mistaken for someone with a real problem.
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Speaking to podcast Celebrity Catch Up: Life After That Thing I Did, he said: "I met Mark early after the first two seasons at an HBO party.
"No one had really seen me as I was doing theatre, so they assumed I was somebody they found in Baltimore and gave them a shot.
"We were at the food line and Mark came up to me and said: 'I love the show, you’re doing a great job man, keep up the good work. Stay off the stuff, you can make a career out of this'.
"And I was just like: 'What are you talking about man? I’m not an addict for real…' And he was shocked."
The Wire, which also starred Dominic West, Lance Reddick, Wendell Pierce and Idris Elba, detailed the Baltimore drug scene with each season focussing on a different aspect - including the dealers, police and shipping port staff.
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Royo added that it was a common occurrence for people to mistake him for an addict as he stayed in character while on set - leading to real drug dealers approaching him on the street to give him drugs and security barring him from entering the set, believing he'd walked in off the street.
President Obama was such a fan of the show, Royo also revealed the former US leader once interrupted his own public speech while on an official visit to the Czech Republic to call out to him in 2009.
Royo, who starred in The Wire from 2002 to 2008, said: "We were filming Red Tails and we had a day off and Obama was speaking. We were standing on the corner with the rest of the crowd and he sees me and he’s like, 'Hey there Bubbles, what’s going on man? Love your work!'
"Everyone flipped out!"