I think there's some commonality in the appeal of Steve Martin and Robin Williams. It's hard to be funny all the time, and tastes change in comedy. However, Martin and WIlliams (along w some other comedians) have some great acting abilities, which when used properly, can generate pathos. Charlie Chaplin thrived on pathos (generally a formula where the first half of movie showed him a lovable clown, the second half a sympathetic figure w noble cause). Williams was particularly good at pathos (Good Morning Vietnam, Mrs Doubtfire, Dead Poet's Society) and Martin his fair share (Planes/Trains, Roxanne, Lonely Guy, even The Jerk). Both were in some comedy bombs, but not exactly their fault (they were popular and overused). Both were great standups in their day.

Someone I thought was outstanding in the pathos formula was Cheech Marin in "Born in East LA". Its amazing to me that he was so underutilized in that role.