Powerful. Thundering. Colossal — puny words to describe Forest Whitaker’s king-size, Oscar-ready performance as Ugandan dictator Idi Amin. There is pow to spare in this first feature — and a lively one — from documentarian Kevin Macdonald (Touching the Void). Whitaker brims with charm and outrageous humor, making Amin’s emerging savagery scary as hell. Based on the novel by Giles Foden. the film views Amin through a fictional character, the young Scottish doctor Nicholas Garrigan (gifted newcomer James McAvoy), who signs on in 1971 as Amin’s personal MD. The script, by Peter Morgan (The Queen) and Jeremy Brock (Mrs. Brown), asks us to see ourselves in Nicholas, seduced by Amin’s charisma and awakening too late to his evil. The plot turns perilously Hollywood when the doc gets it on with one of Amin’s wives (Kerry Washington). But Whitaker is on fire, and as long as he’s onscreen. King keeps you riveted.