I expressed an opinion.
Of course, it isn’t a universal rule, see Dolph Lundgren, etc etc.
* I don’t care if the actor delivering an environmentalist message in a movie is actually good at science for the same reason I don’t care if Keanu Reaves knows king fu.
I've confirmed that both writers of the movie graduated high school, and one of them even graduated college.
I agree with the part about preaching, but fair is fair: they were preaching scientific consensus. They preach what is said by the overwhelming majority of active scientific researchers in this field.
You didn’t say they were wrong I agree, but still .. they were (/ are) right. And why should they be perfect, anyway? They are who they are, flawed and all, but they are right about this and they were right to make that movie and they were right about people being selfish.
Ironically you could say that we are now basically reenacting the movie, proving its point. There’s an asteroid heading for us and here we are, judging the high school grades of the people telling us about its trajectory.
I thought it was very depressing and surprisingly self reflective and poignant in that sense.
It wasn’t a documentary, and even if it were a documentary, a dreadful, preachy, insipid movie that is technically right is still bad.
(I say “technically right”, because let’s not forget that this film was supposed to be a satire.)
The main character (played by DiCaprio) is also depicted as a quite flawed and vain human being as well.
Also honestly, who doesn't feel frustration at the whole real-world situation the movie is actually about?
Your assumption that actors (and writers, those where the ones “preaching” more than the people on screen) have failed highschool at a higher rate than the general population is, I think, rather flawed¹. There are some very bright people in the entertainment industries for one reason or another (doing what they enjoy, and presumably are good at, instead of something else they are good at, being a common situation, there being more money in stardom being another).
Hence a number successful stand-ups who have degrees (in the sciences, not necessarily “media studies” before someone pipe up with that), PhDs, law certifications, and such.
Hedy Lamarr is the best known poster child for this, but too many think she is a singleton exception rather than an indicator that we shouldn't make too many assumptions about what acting talent might imply about other mental abilities.
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[1] And, in fact, more snobby than the film you are critiquing as being snobby!