As for the celebrity impersonations in the new movie The Butler, about a White House butler from 1952-1986:
Robin Williams looks rather like Truman, but plays Eisenhower. (Perhaps Williams was signed to play Truman, but then got reassigned when Truman got cut to shorten the movie?)
Boyish romantic comedy leading man James Marsden plays JFK even though he looks more like RFK.
Minka Kelly, actress/model/whatever and former girlfriend of Derek Jeter, is too va-va-voom to play Jackie Kennedy. She’d be well cast in a historical bedroom farce where Jack has to sneak Marilyn out through the White House dressed in a dark wig and Jackie’s wardrobe (if you want to write this up, be my guest.)
Liev Schreiber isn’t too far off in size and looks from LBJ, but as a Northeastern bourgeois, he lacks the massive Texan personality. Matthew McConaughey of Uvalde, TX is moderately tall and would have been more fun.
John Cusack is Nixon, although he looks more like Kennedy. Cusack plays Nixon as sinister and crazy, like a charmless version of the Nixon/Dracula mélange invented by Jeremy Irons to portray Claus von Bulow in Reversal of Fortune.
Ford and Carter are relegated to newscast footage of themselves.
Jane Fonda is well-cast as a gracious Nancy Reagan. Ideology aside, they are similar enough that they can be compared: – Jane was a better movie actress, Nancy was a better big man's wife (Jane seemed to be a bit of a distraction to the careers of Tom Hayden and Ted Turner.)
Alan Rickman looks surprisingly like Ronald Reagan, but the introspectively gloomy Severus Snape sounds less like Reagan and more like the actor-turned-President doing an amusing impression of Alan Rickman after a White House screening of Die Hard.