My exposure to him began with Unforgiven & through the 90s I flipped back & across his 70s & 90s movies & could hardly ever say I saw a single performance where he was "dialing in" his performance. Thanks in part to him, very few of his movies have I rated under 6/10 (I don't give high scores, about a dozen movies out of 7K+ watched have got 10/10, maybe ~100 get 8 or 9 on 10 - majority non-English, eg have rated most of the 25+ Avengers movies b/w 5-7) - except when he got involved in 'sequelitis' (French Connection & Superman sequels). This doesn't happen for most actors, there's always a few movies that I find mediocre for nearly everyone.
My favorite movie/performance of his is The Conversation (rated 9/10) - also one of my fav movies of all time. Its a movie people nowadays with social media/FX heavy action movie overkill related ADD will find difficult to sit through esp the first half.