Also, most people think of him purely as a comedic actor, but he was also a surprisingly great serious character actor as well.
I loved Airplane and Naked Gun as much as everyone else, but I think that one of his stand out performances, for me, was an absolutely brilliant and charasismatically creepy portrayal of the incredibly effective and unsettling villain in the "Something To Tide You Over" segment of the original Creepshow movie.
His acting, and that character which he very much made his own was definitely one of biggest highlights in the film. There's not too many actors that I can say, are able to make something as simple as whistling "Camptown Races" seem genuinely disturbing and sinister, as well as someone who had a gift for comedy as is well known.
He was an incredibly talented, classically trained, very amazing actor who possessed a distinct charm, a profoundly effective sense of timing and subtlety and took his craft seriously whether he was playing the comedy he was known for, or the serious acting ability that he was sadly underrated for as well.
He will very much be missed.