Two teenage girls, laden down with popcorn and cola, spot a poster for the new film version of Jane Austen's Pride And Prejudice as they cross the foyer of the cinema. They eye up the picture of actor Matthew MacFadyen, who plays Mr Darcy, and one says to the other: "He won't be as good as Colin Firth".

These are not the words that MacFadyen, former star of BBC1's Spooks, will want to hear. Doing interviews for the movie, he skirts round the issue rather like classical actors do when quizzed about the influence of previous Hamlets or Macbeths on their interpretation of a Shakespearean role. They make out it doesn't matter, whereas clearly it does.

As the remark of those girls demonstrate, audiences aren't so good at forgetting the past and are often prejudiced against a performance they haven't even seen.

MacFadyen's Mr Darcy will undoubtedly be compared to Firth's, which made such an impression in the BBC serialisation a decade ago. His performance set female hearts aflutter and made him a sex symbol.

Ever since, he's been trying to live down that wet shirt moment, when Darcy emerged dripping from the lake. MacFadyen has no such scene in the new film. Many will judge that he's made the part his own, with a Darcy who is dark and brooding.

"I grew up watching Colin Firth in films like Tumbledown. To be Mr Darcy after him puts me in very good company," he says.

"I approached Darcy as I would any other part. If you started thinking about it, then you'd never play Hamlet for example. You'd never play lots of parts if you started worrying about who's played it before you. It's the nature of it, you just get on with it."

The actor hadn't read Austen's novel before taking on the role. Nor had director Joe Wright, who made the film "without considering the previous television version and the film version made in 1940". That had Laurence Olivier as Darcy, and Firth's performance had to stand comparison to that.

MacFadyen appears to have got it right. His Darcy is distinct from Firth's although unlikely to win him heartthrob status. "He's no 18th century sex god like Colin Firth and ardent Firthians will baulk at him, but there's no denying that he grabs the role and makes it his own," a critic (female) wrote in an early review.

He isn't the first actor, and certainly won't be the last, to suffer comparisons with past performances. On stage, it's easier to overcome because the work is not seen by so many people, while film and TV versions will survive a lot longer.

If MacFadyen looks likely to get away with being compared to Firth, others before him haven't been so fortunate. The way someone says just a couple of words can prove a handicap to those following in the same role. A simple exclamation of "A handbag" uttered by Edith Evans has proved a stumbling block for every actress subsequently playing Lady Bracknell in The Importance Of Being Earnest.

Mostly, actors suffer more than actresses. Few will bother to choose between Jennifer Ehle's TV Elizabeth Bennet and Keira Knightley's film version as they will between Firth and MacFadyen.

Sometimes the role is bigger than the actor. Time Lord Doctor Who's ability to regenerate himself allows actors to come and go as they please. You might prefer Tom Baker to Christopher Eccleston, but that doesn't harm the project like a flat rejection of MacFadyen's Darcy would.

Sherlock Holmes is another strong character whom audiences go to see rather than the actor playing him. A succession of actors as Batman - Adam West on TV and Michael Keaton, Val Kilmer, George Clooney and recently Christian Bale on film - have all been accepted, perhaps because they're hidden behind cape and mask much of the time.

James Bond is a brand that can make an actor famous but equally allow new people to pick up his licence to thrill. For many, Sean Connery remains the best 007. George Lazenby and Timothy Dalton failed to impose their stamp, yet Roger Moore and Pierce Brosnan both have their share of followers. So one role can be inhabited successfully by different actors.

Some are so closely associated with a role that it's criminal to cast anyone else Dirty Harry without Clint Eastwood snarling "Make my day, punk" or Rambo without Sylvester Stallone being macho are unthinkable. Just as anyone else in that dirty raincoat wouldn't compare with Peter Falk's Columbo.

When producers ignore this rule, as they often do when transferring TV shows to the big screen, they pay the price. Patrick Macnee's John Steed was partnered by Honor Blackman, Diana Rigg and Linda Thorson in The Avengers on the small screen. Giving him a new sidekick - played by Uma Thurman - in the film wasn't a problem, but casting Ralph Fiennes as Steed was. With or without a bowler hat and umbrella, the Oscar-winner couldn't hope to make people forget Macnee's definitive performance.

The film was a deserved flop. More recent TV remakes have fared better despite dispensing with the original actors. Starsky and Hutch, The Dukes of Hazzard and Bewitched haven't earned the ridicule that greeted the film of The Avengers.

Delaying the release date of the new Pink Panther film isn't a good sign for a project that seemed doomed from the start. No-one could hope to scale the comic heights that Peter Sellers did as bumbling French policeman Inspector Clouseau. Steve Martin should have known better than to try in the new movie.

The idea is as silly as remaking Alfred Hitchcock's Psycho frame by frame as director Gus Van Sant did in 1978 when Vince Vaughn's mad, bad and dangerous to know Norman Bates was no match for Anthony Perkins in the superior original.

The list of Hollywood's mistakes with remakes goes on and on. Think Richard Burton and Sophia Loren instead of Trevor Howard and Celia Johnson in a TV remake of Brief Encounter. Or Tom Hanks for Alec Guinness in The Ladykillers, Neil Diamond for Al Jolson in The Jazz Singer, and Faye Dunaway for Margaret Lockwood in The Wicked Lady.

Not everyone fails in changing places. Yorkshire vet James Herriot was played by both Simon Ward and John Alderton in film versions of the BBC series without anyone complaining.

Vine-swinging Tarzan changed his appearance over the years, but as long as he had muscles and a loincloth, no-one was too bothered.

Similarly, Tom Clancy's CIA analyst Jack Ryan was played twice by Harrison Ford - in Patriot Games and Clear and Present Danger - but nobody kicked up a fuss when Ben Affleck took over the character in The Sum of all Fears.

After the exhaustive search for an actress to play Scarlett O'Hara in Gone With The Wind, you'd have thought they wouldn't want to do it again. But the makers of the TV mini-series sequel, Scarlett, searched high and low before signing Joanne Whalley as the Southern belle

Undeterred by his lack of success as 007, Timothy Dalton followed Clark Gable as Rhett Butler. One reviewer, writing on an Internet site, considered him outstanding but wasn't too keen on his leading lady. The most disturbing thing about the new Scarlett was her teeth, she asserted. "Don't get me wrong, her teeth were perfectly straight and white, but she lacked a pretty smile. What made Scarlett so endearing is that she could make a man melt with a smile."

You wonder if MacFadyen will be smiling if the reviews of Pride And Prejudice insist on comparing him with Firth.

* Pride And Prejudice (U) opens in cinemas on Friday.