Manson portrayal: Grade A Ham.
Jeremy Davies played Manson like a Warner Brothers cartoon character. I don't see much difference between Davies interpretation and the satirical Manson who was shown on Saturday Night Live. All the whispery talking and the weird little dance/walk he does in every scene... Sheesh... I bet Davies has a tattoo on his butt that says "Hillshire Farms" he's such an over-cooked ham.
I'm not a fan of Manson but I have seen enough interviews and live footage of him to know that Davies' characterization is utterly farcical. (Example: https://www.youtube.com/watch?v=T0BFZiKe4i0) True, Manson does act very strangely at times, but that crazy routine seems to be a deliberate act he puts on when he feels outgunned by the person he is talking to. Manson has even admitted this.
Davies isn't playing the "real" Charles Manson. He is playing the caricature version that 14 year old kids imagine the first time they hear a story about that weird hippie guy who had a killer cult in California in the '60s and which has become the standard public image of Manson.
Railsback was better, but even he was too over-the-top. I haven't seen any other Manson movies, but I doubt anyone will ever give a true-to-life portrayal of him. Ironically, the public wouldn't want to see or believe anything but the outrageously exaggerated cartoon Manson.