An expression meaning 'hearing about something is very different from seeing it'—seeing is believing; experience matters.
動画で見ただけだと簡単そうに見えるが、聞くと見るとは大違いで実際にやると難しい。
It looks easy in the video, but hearing about it and actually seeing/doing it are very different; it's hard in practice.
現地に行って初めて、聞くと見るとは大違いだと分かった。
Only after going there did I realize how different hearing about it is from seeing it.
君が言うのはいいけど、聞くと見るとは大違いだから自分で確かめてみて。
What you say sounds fine, but hearing and seeing are very different—check it yourself.