"Obviously, I knew Alan very, very well," Helen Mirren, who plays hard-baked Colonel Katherine Powell, told PEOPLE. "I did a play with him, Anthony and Cleopatra, and we went through all the unbelievable difficulties and the passion of that."
She continued: "The man you see on the screen – and that's what I love about it really – is Alan: the wit, the urbanity, the sophistication, the intelligence, the humanity."
Rickman died in January at age 69 after quietly battling cancer. It seems news of Rickman's death was as unexpected for the cast as it was to the world.
"It came as a horrible shock to everyone," Aaron Paul, who plays drone pilot Steve Watts, said Wednesday. "We lost one of the most brilliant people in this profession."
"I didn't know if he was ill, and I don't know if he knew," offered director Gavin Hood of Rickman, who played Lieutenant General Frank Benson in the high stakes drama, which is dedicated to the late star. (Although the film is his last onscreen role, Rickman also lent his voice to the forthcoming Alice Through the Looking Glass.)
"I was so looking forward to seeing him in Toronto [during the film festival]," Hood continued. "And he wrote me a sweet note: 'Gav, I'm so sorry, I'm just not feeling so good and I'm not going to be there. I just need to put my feet up for a few days.' When I think of that now, it absolutely breaks my heart."
According to Hood, Rickman later came to Los Angeles with A Little Chaos, a film that he directed. "We hung out afterward, had a chat, drinks. He was on his feet, gave no hint that he was ill. He was dignified and warm and kind to the very end. And not long after that he passed away.
"It hit me hard, because I read it in the paper," he said while choking up. "It was horrible. It was so sudden ... He was a good man; he really was."