I think I've figured out why Joaquin Phoenix didn't win the Oscar for his performance as Johnny Cash, while co-star Reese Witherspoon was recognized for her portrayal of June Carter Cash. It wasn't that he wasn't terrific: he was. He delivered completely in a tremendously demanding role -- not just the singing, but singing as the iconic Man in Black during various phases of his life.
Part of it is that Phoenix is a very talented actor, and we expect that kind of range from him. Witherspoon had done lighter fare like Sweet Home Alabama and Legally Blonde 2 shortly before Walk the Line, and I don't think a lot of people anticipated the versatility she displayed.
Also, the year of her win, these were the nominees:
Judi Dench – Mrs. Henderson Presents as Laura Henderson
Felicity Huffman – Transamerica as Bree
Keira Knightley – Pride & Prejudice as Elizabeth 'Lizzie' Bennet
Charlize Theron – North Country as Josey Aimes
Charlize had won just two years before for Monster; Judi Dench is a perennial nominee and past winner; Kiera Knightley was in that young-actress-to-watch state that we've seen others go through, like Kate Winslet: you know they're talented, and that it's probably just a matter of time, but the Academy doesn't feel they've quite yet paid their dues ... that left Felicity Huffman and Witherspoon. First-time nomination for both; Huffman had done more television, and Witherspoon was one of the highest-paid actresses in Hollywood at the time in a more popular film about a well-loved country star.
Now compare the competition for Best Actor:
Philip Seymour Hoffman - Capote as Truman Capote
Terrence Howard - Hustle & Flow as Djay
Heath Ledger - Brokeback Mountain as Ennis Del Mar
David Strathairn - Good Night, and Good Luck as Edward R. Murrow
Yeah. Way tougher.
I've now seen two out of the five nominated performances. I've just bumped Capote to the top of my queue, because if Hoffman beat Phoenix as Cash, and Heath Ledger's bravura, haunting performance as Ennis Del Mar (it was his first nomination, so he was in one-to-watch purgatory at the time), then his work really must be something to see, because with competition like that, it doesn't seem an opportune time to just recognize a character actor who has displayed consistently strong work. Nope, with performances like that, I think you'd really have to bring it.
Phoenix has been nominated twice. Johnny Depp has been nominated three times. Edward Norton has been nominated twice.
Let's look at 2009's nominees:
Jeff Bridges - Crazy Heart as Bad Blake
George Clooney - Up in the Air as Ryan Bingham
Colin Firth - A Single Man as George Falconer
Morgan Freeman - Invictus as Nelson Mandela
Jeremy Renner - The Hurt Locker as Sgt. William James
With the exception of Renner, every single one of the male nominees is 50 or older. I'm just saying that with the guys, not counting the recent exception of Jamie Foxx, the Academy seems to prefer a little age on the recipients.
For reference, here's the Best Actor list from Wikipedia.
Side note: Robert Patrick was also good.