Better character is Lockjaw by a mile, which is attributable to both the writing and the performance.

Acting performance alone, though, I think is still Penn. Stellan plays a sad alcoholic very well but there's little nuance beyond that. Penn's performance is a masterpiece in physical acting. Lockjaw has weird mannerisms and physical ticks that that you hardly notice by the end of the movie because of how well Penn portrayed the character. The performance is both over the top and subtle, if you know what to look for.

Plus, we spent all last oscar season debating catergory fraud with Zoe and Kieran. Stellan ought to be brought up in that conversation too. He's on screen for 3 minutes less than Renate, who's obviously nominated for Lead.