[jtr]

Jack Lemmon is a god, anything he does must be subtle. But yes, I see your point, non-verbal acting can look pretty hammy when you stare at it for a while.

This is actually a tough question. Otnemem rightly points out that many people are naming favourite moments rather than subtle moments. At the risk of blurring the line again, I nominate the scene in "You Can Count On Me" when there's an exchange of glances between brother and sister Mark Ruffalo and Laura Linney, and it's clear that they're referencing the film's title as something they used to say to each other as kids, but there's no use of the actual words. I thought that was pretty subtle.

[Cue DrewDevil to appear and tell me I suck at liking movies again, I guess.]