There's a special frisson when a performer renowned for comedy goes straight. I didn't feel it when the stand-up comic John Bishop [2] popped up as the hero's murdered chum in the flashback scenes during Ken Loach's Route Irish, but that's only because a mixture of luck and careful planning has enabled me to avoid that species of TV comedy show on which Bishop has appeared -- Live at the Apollo, 8 Out of 10 Cats, and all the other ones that you can watch for three or four minutes before the urge to grind broken glass into your eyes becomes irresistible.

I recognise his larky lad persona, and I can see why Loach used him: Bishop emphasises the present-tense chirpiness, rather than making the character a living portent of the tragedy which is to befall him. Besides, the director has a history of looking to the stand-up circuit and the working men's clubs of the north for his actors, among them Bruce Jones [3] (star of Loach's Raining Stones, and later a Coronation Street regular) and Crissy Rock [4] (who played the lead in the harrowing Ladybird, Ladybird).

With more established comics who bring the baggage of their previous work, the change of tone can be jarring; it may not always work, it may even capsize the film, but there's a unique tension for us in watching a performer from whom we expect warmth or humour, only to find those qualities absent or mangled. Maybe Stanley Kubrick was just having a terrific wheeze when he entertained the idea of casting Steve Martin in the role that eventually went to Tom Cruise in Eyes Wide Shut. But he was undoubtedly prescient -- that was back in the late 1970s, when Martin's only leading film role had been in his delirious vehicle The Jerk, yet Kubrick must have seen in his mania something fraught that could be used to serve dramatic material. Sure enough, Martin was outstanding a year or two later in the rhapsodic movie version of Dennis Potter's Pennies from Heaven [5].

Cut to a decade on and other directors (along with Martin himself) came round fully to the idea of him as a "straight" actor, with the comic playing earnest in Grand Canyon, untrustworthy in Leap of Faith and subtly chilling in David Mamet's The Spanish Prisoner [6]. Is it the proximity of comedy and aggression (comics "slay" or "kill" their audiences, after all) that makes it strangely appropriate when comedians not only turn serious, but get sinister with it?

There are rebranding opportunities, to be sure. Robin Williams seemed pretty much spent as a comic actor before he relaunched himself as a nasty piece of work in a toxic hat-trick of movies (Death to Smoochy, Insomnia, One-Hour Photo). Those pictures prompted half the audience to marvel that they had no idea he could be so creepy, and the other half to exclaim: "So you didn't see Patch Adams, then?" Even if that defection to the dark side didn't stick, its residue could be felt recently in Williams's work in the pleasingly unpleasant black comedy World's Greatest Dad.

Likewise, Adam Sandler's career didn't take a turn for the avant-garde after Paul Thomas Anderson cast him as a wuss with a temper in Punch-Drunk Love [7], but it's nice to think it let some fresh air into his persona, and allowed him to accept that he didn't always have to be the guy with whom moviegoers would most like to have a pint (see Funny People for another example of how fine Sandler can be when he's not sending valentines to the audience).

It must be galling that comedy is so rarely rewarded or even acknowledged by the award-winning bodies. Despite more than 20 years of devilishly inspired comic performances, Bill Murray never got within sniffing distance of an Oscar until he landed a Best Actor nomination for his melancholy work in Lost in Translation. I wouldn't claim Dan Aykroyd as Murray's comic equal but it's striking that he too was noticed only once he'd notched up some respectable, "real" acting in Driving Miss Daisy.

Will Ferrell has voiced his irritation with this divide between comedy and drama, where all the recognition goes to the latter. He put his case most eloquently in a song [8] he performed with Jack Black and John C Reilly at the 2007 Academy awards ceremony. It began with Ferrell reciting: "A comedian at the Oscars/ Is the saddest man of all/ Your movies may make millions/ But your name they'll never call," and ended with him resolving to play "a guy with no arms and legs/ Who teaches gang-bangers Hamlet." Somewhere in between, he imagined dining with Jeremy Irons, then threatened to break Ryan Gosling's hips.

As with most comedy, it was deadly serious in intent. He told me: "I don't think the producers of the show even got what we were doing. They were backstage saying, 'Oh, that was lovely. Very funny.' They didn't realise every word was true."

Comedians are bitter and unhappy. This much we know. But it's rare that they reveal that side of themselves on screen. When they do, it can be deeply unsettling, chiming as it does with those moments in childhood when a cherished and apparently good-humoured parent shows a darker aspect to their character. Sometimes the sourness is already so present in their comedy that it isn't hugely jarring to find that it translates well to drama -- was anyone really surprised that Murray could play a bullying, insecure mob boss in Mad Dog and Glory?

That familiarity isn't always a shock absorber. I wonder if I will ever forget the sight of James Bolam, a fixture from my childhood thanks to Whatever Happened to the Likely Lads?, playing an avuncular pimp in the gloomy British film Stella Does Tricks. Bolam's appeal was always mildly unsavoury, but it was inspired casting to imagine him as this predatory monster. He plays it like a cross between Michael Caine in Mona Lisa, Alan Alda in Crimes and Misdemeanours and Robert Mitchum in The Night of the Hunter. He oozes ugliness; after his scenes, you feel like scrubbing yourself clean. Part of the disgust is inevitably bound up with his past life as a Likely Lad. You look at him and you think: "Oh, Terry..."