The Best Supporting Actress race is notoriously difficult to call. The category’s more prestigious older sister, Best Actress, tends to grab all the headline-making transformations along with the big-name fashion icons making good on years of empty fame. Supporting wins, on the other hand, can serve lesser-known favorites such as a surrogate Lifetime Achievement Award (Judi Dench, “Shakespeare in Love” 1999), a welcome-to-Hollywood anointing ceremony (Angelina Jolie, “Girl, Interrupted” 2000), an apology for past oversight (Renee Zellweger, “Cold Mountain” 2004) or just another way of honoring the year’s favorite movie (Jennifer Connelly, “A Beautiful Mind” 2002 and Catherine Zeta-Jones, “Chicago” 2003).

With no clear-cut definition of what, exactly, makes a Best Supporting Actress winner in a field of all-around superb talent, 2005’s Supporting Actress competition is this year’s definitive Oscar-betting spoiler, as well as one of the most exciting reasons to watch.

Typically, there’s at least one nomination in every category who appears out of nowhere — and for “Best Supporting Actress,” her name is Sophie Okonedo. Playing the wife of Don Cheadle’s Paul Ruesasabagina in “Hotel Rwanda,” Okonedo was certainly very effective. More importantly, “Rwanda” is the most politically important film competing this year. Here is an event in which a million people were brutally killed while the Western world turned its eyes away. Because Cheadle has no chance against Jamie Foxx, and the Academy loves to pretend it cares, a win for Okonedo wouldn’t be entirely plausible. Her relatively unknown status, however, and the limited development of her character, not to mention the fact that she is competing in a tough field of big-name talent all make the win improbable.

Also unlikely to take home the Oscar is Laura Linney. She was stunning as the wife of famed sex researcher Alfred Kinsey in the largely overlooked biopic, “Kinsey,” but, like Okonedo, she played the wife character in a movie focused overwhelmingly on the husband. Linney certainly has her fans, but she’s not a celebrity powerhouse, her movie received almost nothing else in the way of Academy or box office recognition and she hasn’t had an Oscar stolen from her yet. Though the strength of her performance warrants her an Oscar, she’s not likely to win it this year.

Virginia Madsen’s small role as a wine-loving waitress and the object of Paul Giamatti’s affection in Alexander Payne’s poignant and lyrical “Sideways” was among the most elegant and unforgettable performances of the year. Madsen transformed a brief and thinly written paper role into a complex, fully-fleshed human being, hitting every emotional note with perfect clarity. “Sideways” marks a remarkable comeback for Madsen, which would be nicely capped by an Academy Award. On the strength of her performance alone, she deserves it more than any other competitor. That being said, this is a movie that seems to have peaked in hype at least a month ago, and lead actor Paul Giamatti’s omission from the Best Actor category doesn’t bode well for the chances of his co-stars. On the other hand, Academy voters may feel that an award for “Sideways” in this category is an award for the whole cast.

Probably the most famous nominee, Natalie Portman’s chances in this category look exceedingly good coming off a Golden Globe win. Portman played an alluring and mysterious stripper with spectacular grace and powerfully understated emotional depth — the first of many signs in “Closer” that the whole parade was completely unreflective of reality. However, the fact that she wasn’t even nominated for a Screen Actor’s Guild award is a major red flag. The Guild is comprised of mostly Academy members who tend to duplicate their votes, making it the most reliable Oscar predictor. Nicole Kidman also failed to pick up a SAG nomination for “Moulin Rouge” in 2001 and went on to lose the Best Actress race.

And though Portman deservedly earned the biggest raves for “Closer,” the movie garnered a lukewarm reception from critics and was mostly reviled by audiences. Still, the also gorgeous then-starlet Angelina Jolie won for another indifferent little-seen movie, and voters are likely to consider Portman’s work in “Garden State” as well, making her odds in this category extremely strong.