That's because you're stupid.

I laughed at that scene along with many other scenes. He was so happy to have the apple because he probably hadn't eaten in 3 days. Where was he getting his food? This film is so unrealistic I can't get any meaning from it at all.

That scene was annoying to me, and now you remind me of it.
People only want to hear what they want to hear

I also thought it was improvised. I'd like to think that was Emile's reaction after Sean said "cut!" and he later decided to keep that sweet smile to the camera

That was one of my absolute favourite scenes of the film! It seemed so improvised. It did seem a little weird how he looked into the camera and broke the fourth wall, but I dug it!

McCandless did in fact take "selfies" all along not video, of course, but he would set up the camera with a timer and go and pose for pictures. Penn's scene was simply an animated depiction of that.