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The actor, who has been dating the actress off-and-on for several years, insisted that Barrymore appeared "more beautfiul" to him when he saw her professionalism and kindness at work.
Speaking to Us Weekly, the 32-year-old said: "She's so funny, insanely funny. But I also got to see a side of her during this movie, which I hadn't seen, because I'd never known her in a professional capacity.
"I got to see how she treated the people that she's working with and it made her so much more beautiful. She's lovely to everyone - to PAs, studio people, no matter where you are in the hierarchy. It's rare."
Long recently revealed that he and Barrymore have battled many obstacles to keep their romance alive.
Going The Distance debuts in US cinemas on September 3.
