Superheroism is no laughing matter.

It's a fact that's easy to forget after dull-as-dishwater turns in the likes of Troy, The Other Boleyn Girl, and the new Time Traveller's Wife, but it's a fact nonetheless: Eric Bana used to be funny. He was a stand-up comedian and TV sketch trouper before his breakout role as notorious Australian criminal Mark "Chopper" Read in Chopper. Bana is an absolute riot in Chopper, playing the part of a violence addict with fearsome charisma and hilarious obliviousness to the costs of his horrible actions. When Bana headbutts his girlfriend and her mother into unconsciousness and says to the knocked-out girlfriend, "look what you did, you upset your mum!" his voice has a childlike earnestness that makes it impossible not to laugh. Film fans had every reason to expect a lot more vibrant, funny performances from Eric Bana after that.

Then he came to Hollywood and became the biggest stiff this side of Christian Bale. Nowadays, a Bana performance involves a shit-ton of glowering, looking anguished and grimacing when he's not glowering. At any given moment on screen, he looks as likely to crack a smile as he is to take a shit on the camera lens.

What the hell happened?

The answer can be found by examining the parallels between Bana and other gifted comic actor who lost his light touch, Michael Keaton. Like Bana, Keaton got his start in show business as a stand-up comedian. After his big break playing the sleazy pimp/undertaker in Night Shift, Keaton built a career as the best comedic actor of the 1980s. We're talking Night Shift, Johnny Dangerously, Mr. Mom, Gung Ho and, to top it all off, Beetlejuice. He made a couple of less-than-stellar comedies in the '90s, but the end of the Reagen-era pretty much meant the end of Michael Keaton as a go-to guy for comedic roles.

What the hell happened? The same thing that turned Eric Bana from a live-wire funny man to an epic stick-in-the-mud: he got turned into a superhero.

You just can't argue with the chronology. Keaton: Gung Ho, Beetlejuice, BATMAN, My Life, Desperate Measures. Bana: Australian sketch comedy shows, Chopper, THE HULK, Troy, Munich. There's something about accepting the burden of iconic superherodom that sucks the joy out of actors. The effect isn't universal. It seems to be more noticeable on actors who had reputations for light entertainment before they donned the colored tights. Val Kilmer has always oscillated between being a stiff and being the loopiest motherfucker on earth, and Batman Forever didn't do a thing to change that. It's just too bad that stiff-ass Val had to show up for that one. George Clooney was still trying to figure out how to act without tipping his head forward in every take when he made Batman and Robin. And Tobey Maguire has always, and will always, be a mopey dink, costume or no. Whatever the mechanism, the responsibility of carrying a multi-million dollar film franchise and embodying a character with deep roots in popular culture just sucks the fun out of actors more used to the lower stakes of comedy. Then, even when they move on to less mythic characters, that ponderous sense of gravity and humorlessness lingers in their work. Playing a superhero, it's the STD of film acting.