Speaking to Digital Spy, the Welsh actor admitted that he was stuck for words when he and co-star Yasmin Paige first met the Zoolander actor.
"It was really awkward, actually," Roberts said. "Obviously he's Ben Stiller so he walked in the room with his aviators on... very cool. He walked in and I was like, 'I saw Greenberg, well done!' I didn't know what to say!"
Paige noted that Stiller spent his time "in the sunshine in Hollywood" while they were working on the Richard Ayoade-directed film in Wales.
Roberts added: "He actually sent a video saying, 'I'm in Hollywood. I'm not coming out because it's raining.'"
Submarine debuts in UK cinemas tomorrow.
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