Ron Howard was in disbelief about the death of his Happy Days co-star, Erin Moran.

Officials told ET that the 56-year-old actress likely died due to complications of stage four cancer. On Monday's episode of CBS' The Talk, Howard admitted that the death of his former onscreen sister hit him hard.

WATCH: Bill Paxton Dies at 61 -- Tom Hanks, Helen Hunt and More Celebs Pay Tribute to Actor

"It’s been a rough time. Bill Paxton, we lost a while back -- a very good friend," he said. Paxton -- who starred in several of Howard's movies -- died in February due to complications from surgery. He was 61.

"My stepmother, Judy, passed away a couple of months ago," Howard continued. "So, the Erin event really did hit hard and shock. I had no idea she was ill whatsoever.”

Howard and Moran played siblings Joanie and Richie Cunningham on Happy Days, and the 63-year-old actor-director has fond memories of working with her. "She was always the kid on the set, in a way,” he reminisced. “When I think of her then, which is how I choose to think of her, she was this feisty, spirited little girl."

Howard also recalled a Happy Days rehearsal that turned into a messy, shaving cream fight. "It was kind of a boys club, we had all the guys goofing around,” he said. "There was a lot of hi-jinks and goofy frat stuff going on. But I do remember a particular moment where we were in a rehearsal, in our rehearsal clothes, and somehow it was kind of turning into a shaving cream fight. And she did sneak up behind Henry Winkler with a pretty good shaving cream pie, and she got him."

WATCH: Happy Days Actress Erin Moran Likely Died of Cancer, Officials Say

Upon hearing the news of Moran's death over the weekend, Howard sent out a heartfelt tweet in honor of his late co-star. "Such sad sad news," he wrote. "RIP Erin. I'll always choose to remember you on our show making scenes better, getting laughs and lighting up TV screens."

Winkler -- who played Fonzie on Happy Days -- also showed his gratitude to Moran's fans. "THANK YOU ALL for your heartfelt and generous messages about untimely and tragic death of Erin," he tweeted on Tuesday. "The Happy Days family will never forget her."

Here's a look at how the actress' other Happy Days co-stars paid tribute to her: