The unconventional upbringing, the struggle to emerge from his brother’s crippling shadow, the career swerve from actor to rapper, Joaquin Phoenix is a lot of things, but ordinary isn’t one of them. Still, his bizarre behavior recently on David Letterman’s show provoked a rash of head scratching. Was the Oscar-winning enigma cracking up? Or just acting? This hour-long A&E Biography probes the past in search of clues.