If Pierce Brosnan's appearances in Mamma Mia! prove anything, it's that there's very little overlap between "talented actor" and "talented singer." Yet at some point in their careers, almost every famous musician in the world will take a shot at the movies. Some of them even make it! But for every Will Smith, Ice Cube, or, um, Mark Wahlberg, there's a legendary singer who proves so bad at acting that even Hollywood wouldn't let them do it. (And they let Shatner try for years!)
Neil Diamond Did Blackface ... In The '80s
The Jazz Singer was the first ever American talkie, so people tend to be very forgiving of its flaws. Sure, the sound was pretty bad. Sure, the dialog was stilted. And sure, the movie was about iconic singer Al Jolson doing blackface to steal work from African American performers. But hey, it was 1927 -- we can't hold people from that time to our standards. The same cannot be said of the 1980s, or singer Neil Diamond, both of whom should have known a lot better.
In the mind-boggling 1980 remake of The Jazz Singer, Diamond plays Jess Rabinovitch, the youngest in a long line of Jewish cantors (singers who only use their voices to praise God). But Rabinovitch doesn't want to be a cantor, he wants to be a cool rock star -- which would've been a bit more believable if Diamond didn't already look like a middle-aged assistant bank director by the time this movie was released. It would've also been more convincing if he managed to bring any of his own megastar flair to the screen. Instead he grunts out all of his lines while, as Roger Ebert put it in his scathing review, awkwardly "looking at people's third shirt buttons" rather than their eyes.