There will be a great many of you who reading this who will, no doubt, remember when Attenborough encountered a group of mountain gorillas in Dian Fossey's sanctuary in Rwanda, which took place in episode 12 entitled "Life in the Trees".
These gorillas were quite used to human contact (I would imagine it would have been virtually impossible to film up close with truly wild gorillas), although even Attenborough hadn't bargained on getting quite as close as he did to a gorilla family. The naturalist suddenly found himself with an adult female gorilla literally staring him in the face, which led to him dumping his scripted dialogue and resorting to going ad lib instead. You can see this in the official BBC video clip above.
Life On Earth first edition (1979) hardback book