Yep, we can totally understand why!
"I remember when I was 15 doing scenes with Ralph for the first time. He was even scarier than Alan [Rickman, Professor Snape] was at first.
"Alan was super intimidating to start off with as well, but then you get into it, but Ralph genuinely scared me for a few years."
These scenes are still scary just looking at them in GIFs, so we totally get why Dan was so scared!