"I think coughing up slugs was quite hard. Ron has a scene where he has to cough up these giant slugs"

Rupert Grint is referring to a scene in a movie or tv program in which his character, Ron, has to spend gigantic slugs. This is likely a funny scene, as it is difficult to envision a situation in which a person would have to divulge slugs in the real world. Grint is suggesting that it was hard to act out this scene, as it was likely fairly unusual as well as uncomfortable. He is also most likely revealing appreciation for his personality's valor in tackling such a difficult scene. The quote indicates that Grint located the scene to be amusing, as he is describing it in an easy going way. It is likely that the scene was favored by audiences, as it is remarkable sufficient for Grint to still be speaking about it years later on.