Two of David Attenborough's career highlights, as mentioned in this Mirror article, are his visit to Komodo back in the 50s and his famous encounter with the Mountain Gorillas in Rwanda for his Life on Earth series.

We went to Indonesia in 1957 to find the Komodo dragon, which had never been filmed.

Today you can get to Komodo overnight, but it took us three months.
[...]
I had no intention of getting close to the gorillas. I was crawling along preparing to turn round then suddenly this thing appeared and put his hand on the top of my head and put his finger in my mouth. Then two baby gorillas came up and took off my shoes.

A lot of it wasn't filmed. The director told me afterwards he thought: "God, the gorilla's going to tear his head off... and we're only half way through the series."