ive just seen The Apartment for the first time, and it has reinforced an opinion ive held for a while:
I find 50's/early 60's era Jack Lemmon to be an irritating screen presence, in much the same way that some people find Dustin Hoffman or Woody Allen grating. In some of his films from this era i just want to grab Jack and tell him to STOP FIDGETING!!!! THE MATERIAL IS DOING THE WORK - TONE. IT. DOWN.
That's not to say i don't think he didn't deepen as an actor later in his career; his performance in Glengarry Glen Ross is one of my all time favourite performances - nobody swears like Jack! But i do find him grating in this and a lot of his other work from around this time.