"The sky above the trees is cloudy, you are under the thick canopy of the trees and you are totally immersed in thick leafy bushes. It is very, very dark and gloomy! You are balanced, wobbling, on a thick mat of dead and dying branches - it feels like a haystack underfoot. Your companions are experiencing the thrill of a lifetime: they have got a mountain gorilla in the viewfinder of their Instamatics and they are not going to move out of the way for you!"
I have since realised that you can buy 1600 ASA film from the big photo suppliers like Jessops. You need to keep it in a fridge as much as possible and you should have it in X-ray bags while passing through airport security. But it will give you two more stops over 400 ASA film. That would mean your fastest-available (about 1/15 sec) shutter speed becomes 1/60 sec. Not fast, but it much more likely to a decent photo than 1/15 sec.
If you are determined, fit and agile then it might be worth taking a good monopod. If it is super solid one then you might be able to cling to it to stop you rocking as you stand on the vegetation.
On the other hand, you may be lucky and meet the gorillas in one of the few clearings in the forest. If so your 1600 ASA film now becomes a liability! So how about a really dark ND filter on the camera? No - given a heaven-sent opportunity like that, you should quickly haul out the 1600 ASA film and replace it with 100 ASA which you had thoughtfully brought in your pocket.
You are lucky - nobody gave me this good advice before I went on the trip. I hope your photos are good.