Example of an actor who is perceived as sucking one one show and, well, sucking less on a show where he was the lead - David Boreanaz. When he was on Buffy, everyone said he was so wooden he was in grave danger of turning into a coffee table.

Heh. For years I argued until I was blue in the face that he was often good on Buffy: at first when he played sardonic and mysterious, then in the second half of season 2 when he played gleefully evil and sarcastic. It was the romance-oriented bits where where he was always looking yearningly at the camera and 2/3 of his lines consisted of the word "Buffy" that came off as flat, generally in mid-season 2 and season 3.