As a critic, I often find that acting is the most difficult aspect of a film to analyze. There are simply too many variables and delicacies. Is the performance an embodiment or an imitation? What is acting and what is overacting? If a performance seems a little wooden, is that consistent with the style of the film?
That's one of the reasons I like Cate Blanchett's performance as the Swinging London version of Bob Dylan in I'm Not There. She makes this performance easy to sum up. She goes up against a couple of the best young male actors of their generation (Christian Bale, Heath Ledger, etc.) and lays waste to them while playing a man. It's ferocious.
The performance is easily worthy of an Oscar. However, in what capacity isn't known yet. In fact, it is rumored to be under debate whether to push her as Best Actress or Best Supporting Actress. That debate is generated by the simple fact that, as one of six actors playing Dylan, she spends, I would guess, 20 to 30 minutes onscreen, short for the traditional idea for a lead performance.
For my money, it's clearly a lead performance. Start with the obvious. She's playing Bob Dylan in a Bob Dylan biopic. Her character is the center of the film while she's onscreen. If she is a supporting actress, whose performance, exactly, is she supporting? The most obvious precedent is Geoffrey Rush's Oscar-winning performance in Shine in which he is one of three actors to play David Helfgoff. No one would suggest that he is anything less than the lead, even if Noah Taylor approaches him in both screen time and effectiveness.
The debate points to one of the lingering Academy Awards issues, one we're not going to solve here today. What is a supporting performance, and what is a lead performance? With the rise of split story structures and observer-narrators, that question is harder and harder to answer. At least one supporting actor winner in each of the last two years has been, in my opinion, a lead (George Clooney in Syriana and Jennifer Hudson in Dreamgirls). Last year, the possibility was there for three arguable mismatches, with two winning (Hudson and Forrest Whitaker, whose Idi Amin is arguably a supporting character to James McAvoy's globetrotting doctor).
Anyway, I wish Blanchett the best, no matter where she ends up. She deserves it.