"Ensemble Cast" is one I struggle with. Does it mean when I think the entire cast is good? Does it only apply to large casts? Is it more about how the actors play off each other rather than evaluating their performances in isolation?
I checked to see if the SAG Awards had any good guidelines here, but all I found was this:
CAST ELIGIBILITY: “A Cast in a Motion Picture” includes all actors whose names appear in the cast credits of the final release print. Motion Picture Casts shall be represented by those actors billed on separate cards in the main titles, wherever those titles appear. In cases of special, unusual or non-billing or credit, eligibility shall be at the sole discretion of the Awards Committee. Members of the cast who are not single card billed but are credited in the cast crawl of the motion picture announced as the recipient of the Outstanding Performance by a Cast in a Theatrical Motion Picture shall each receive a certificate
Which I take to mean that they're main concern is not having to give out too many statuettes.
When I vote for Ensemble Cast, I'm basically looking for three (or more) exceptional performances and no bad ones. I sometimes try to take into consideration the work of the cast director in assembling the ensemble, especially if someone was successfully cast against type. I don't think I would ever consider a cast of just two people (and maybe not even three), but I'm sure a film will come along to challenge that.