In the 2000 mini-series of Dune, I noticed that two of the characters, specificly Muad'Dib (while talking to Lady Jessica, in the desert), and Baron Harkonnen (while talking to Emperor Shaddam) rub their temples alot. I was wondering what the significance of this is? I read that it helps to relieve migraines, but is that the reasoning for it in the movie? PS, I found this, but IDK that that explains it, either.
Baron Harkonnen appears to rub his temple more as a nervous tic than anything else, something he does while thinking. He may genuinely have a bit of a tension headache going on, but that's not really the point. The point is that it's a very visible tic.
Paul does not start exhibiting this tic until after he's gone through the Spice Trance. It's the audience's first hint...and then Jessica's...that Paul has access to all the memories, and mannerisms, of all of his ancestry, male and female. It also serves as foreshadowing of what can happen to a less well formed personality who has undergone the trance--what will happen to Alia later on. If even a strong, fully formed personality like Paul can start exhibiting one of the Baron's mannerisms, what does that mean for the "pre-born"?
All of this is John Harrison's effort to show us what Frank Herbert tells us in the novel. There's no in-story explanation for why the Baron does it (e.g. does he really get headaches? Or is it just something to do with his hands while he's thinking?), because it doesn't come out of the novel at all. It's a detail invented for the mini-series to give us a visual thread to follow.
While I never paid much attention to it, it is probably a hereditary mannerism - Vladimir Harkonnen is Lady Jessica's father and Paul Atreides grandfather.