Fonda's wink....
...just after Cass has been revealed as the murderer, Lincoln approaches the bench, tells the judge he has no more questions and the prosecutor 'Your witness'. He then turns to watch the ensuing hubbub for a few seconds before giving a sly wink to someone offscreen to his right. I can't quite decide whether this was scripted/directed, or whether Fonda momentarily broke character. It's quite discernible when you're looking for it, but it barely registered the first time.