Klugman, best known as a costar of The Odd Couple and later, the lead in Quincy, did not get better with age as most actors do; instead he settled mostly into repetitive overacting.
This was especially difficult to watch in The Odd Couple where, week after week, he diminished the comedic efforts of the inspired Tony Randall. Randall had a much better partner in Rock Hudson, against whom he played in all three Hudson/Day vehicles, to great effect.
Here's a 1957 scene from 12 Angry Men in which Klugman (reputedly a charming and considerate man offscreen) showed Henry Fonda how to use a switchblade. Klugman was better before he was worse.