You know Kate, I watched the clip a few more times and I have to admit you're right. Despite his displays of nervousness during the first verse, he does appear to be singing happily unawares at the moment before he falls never-the-less. What mixed emotions
Actually I can interpret that easily enough.
Imagine this: The three pigs have been ascending for a little while and the one on the end says, "I don't know why I ever let you guys talk me into this."
One of them answers, "Oh come on. You agreed it was a good idea that the three of us go an a hiking holiday together."
He replies, "Yes but I thought we'd stick to the valleys and maybe the itty-bitty hills, but I don't like how far up we are. It'll be a disaster if we fall from this height."
And the other one says "Oh you worry too much. Hiking tracks are perfectly safe. Nothing will happen to us. Tell you what. Why don't we all sing some verses of the traditional hikers song "The Happy Wanderer" as we go. It'll relax you and you'll start to appreciate the beautiful view from up here. We'll even share a line of the chorus each. You can start and you'll have the first Val de ahahahahaha part. Okay?"
Well he reluctantly agrees, though he still peers down nervously during the verse. But while they sing the chorus he starts to feel the spirit of the song and thinks maybe he is being a silly pig and prepares to sing joyfully when...
well you know everything that happens then of course