Wilder died on Sunday (28Aug16), and in a new interview with Variety.com, Ostrum admits he's still recovering from the shock of his old friend's death.
"It's kind of like losing a parent," he said. "You know it's going to happen, but it's still a shock. He was not in good health at the end and it was not unexpected by any means, but when it happens, it hits you like, 'Gene is gone and there will never be anyone like him again'."
Ostrum had not seen Wilder since they made Willy Wonka & the Chocolate Factory together in 1970, but he has fond memories of his co-star, adding, "They would break for lunch and Gene and I would always buy a chocolate bar and share it on the way back to the set."
He also recalled, "He was so quirky. You never knew what to expect from Gene. He never let on how he was going to read a line or convey an expression. That's why the film works, because he made Wonka so unpredictable."
Ostrum's young castmates have previously paid tribute to the late actor, joining famous fans and friends.