As for Michael being funny onlyu in GF III, I beg to differ. Michael is a regular comedian... a really loose and funny guy.
In I he has us rolling on the floor with his "sit down, finish my dinner" routine, and in II his sly way of telling Fredo that Spanish for Bannana Daquari is Bannana Daquari.
In the begining of Godfather I, we see a pretty loose, soft, care free and humrous Michael who gradually changes into this tight, hard, burdened and serious Michael.
In Godfather II we are shown the changed Michael who went from being this loose, humorous and care free guy who's now completely turned into this cold, selfish and power hungry guy. But at times, like the scene with he and Fredo in Cuba, we are given a sort of reminder of the Michael that once was, a small glimpse into the humor that Michael once had.
Then by the time GFIII rolls around, we are finally given the finished Michael, the remorseful guy who still has some of his selfish traits, but who is also turning back into that humorous, caring and soft guy that we saw in the begining of GFI. But unfortunately for him, it's too late.
The young and lonely Michael in the begining of GF I who was basically isolated from his family, by his own hand, completed the circle by now becoming the old lonely Michael, seperated from his family, because of his own deeds, who ended life basically the way that he began it, an isolated man.
Don Cardi