When is a supporting actor not a supporting actor?

Every awards season throws up surprises and snubs, and usually at least one awards body will go off piste and treat an actor's excellent performance quite differently. Simply put, when is a supporting actor not a supporting actor?

Take a look at what's happened so far with the SAG and Golden Globe nominations:

• Rooney Mara nominated for the Best Actress Globe, but SAG have placed her in the Supporting Actress category.
• Alicia Vikander nominated for Best Actress at the Globes, but Best Supporting Actress in the SAG
• Christian Bale is nominated for Best Actor at the Globes, but SAG have placed him in the Supporting Actor category

There have been lots of other similar cases in movie history:

• Was Anthony Hopkins' role as Hannibal Lecter really a leading role and not a supporting one?
• Was Alan Arkin in Argo a supporting actor or a cameo performance?
• Surely Casey Affleck was a lead actor in the Assassination of Jesse James not a supporting actor?
• Judi Dench in Shakespeare in Love was surely a cameo not a supporting role?

Also, the 'Supporting' category is often used/abused to reward an ensemble by highlighting the performer most likely to get nominated: Ian McKellen in Lord of the Rings, for example.

The main cause of these confusions is voter apathy, certainly in the case of the Oscars. Stories are legion of Academy members asking their partners to vote for them, and of late, if you don't send screeners (DVDs) of a film to Academy members, it will not get nominated. So the herd do what they're told: Harvey Weinstein and his team push Rooney Mara for Supporting Actress, when she is clearly in a lead role, and she gets shortlisted in the 'wrong' category.

I say 'wrong' because it's clearly wrong to a film fan, but it makes good business sense for Harvey to ensure his two leading ladies are not competing against each other: surely Carol will make more money if it wins two Oscars for Blanchett and Mara in Best Actress and Best Supporting Actress categories respectively than if the two compete against each other and lose?

How does the Academy define leading and supporting roles? Rather conveniently, it doesn't. Here's what it says: "The determination as to whether a role is a leading or supporting role shall be made individually by members of the branch at the time of balloting."

Furthermore, it notes that if a performer achieves enough votes in a single category for each of more than one eligible performance, only one performance will make the cut. So no hope for Cate Blanchett in Truth then as her role in Carol will garner more votes. Ditto Michael Fassbender: his Macbeth is better than his Steve Jobs, but if Oscar is going to support one of those performances, it will be the Apple biopic.

Of course none of this explains how neither SAG nor the Globes nominated the outstanding female performer of the year, namely Charlotte Rampling in 45 Years...