Donald Clarke is confused by doppelganger movie stars

While interviewing Will Ferrell, star of the chortlesome Step Brothers, I briefly found myself trapped in a mental standoff of my own unconscious devising. Halfway through a question concerning the actor who plays Ferrell's mother, I realised that I was, not for the first time, in the terrible grip of Christine Lahti/Mary Steenburgen syndrome. Argh! Which of these vaguely similar actors played the part?

You are, of course, entirely correct to draw a pointy hat on Screenwriter's photo and inscribe a big "D" within it. Though the two performers were born within three years of one another and have similarly amiable screen presences, they have developed entirely distinct careers. Yet, having occupied the same air in the late 1970s, Steenburgen and Lahti will, I suspect, remain forever manacled together in my addled brain.

Other conditions from which I have suffered include Juliet Stevenson/Harriet Walter illness; Powers Booth/Stacy Keach disorder; Victor Mature/Tyrone Power syndrome; Elliot Gould/ George Segal disease; and Ashley Judd/Angelina Jolie chorea.

These sicknesses are not always terminal. Judd/Jolie offers a classic case study of how Cinematic Confusion Malaise can arise and how changing circumstances can bring about a cure.

Some eight or nine years ago, two female actors, both with the initials AJ, began appearing in a series of supposedly dark supposed thrillers with titles like Tainted Hostage, The Bone Spidersand High Jeopardy. Though Judd is seven years older than Jolie, the performers fought over the same turf, before Angelina, assisted by a useful romantic alliance and a surprising Oscar, elbowed her unacknowledged rival into the wings.

It's easy to tell them apart now. One appears in pictures directed by Walter Z Nobody and Rufus Taxdodge. The other is so covered in gruesome tattoos that only her left eyebrow is still visible.

The variants of Cinematic Confusion Malaise that prove incurable tend to be those associated with two perennial supporting players. As the 1970s progressed, George Segal remained a reliable second banana, but Elliot Gould became a kind of star, and Segal/Gould Disease gradually went into remission. By way of contrast, Juliet Stevenson and Harriet Walter remain only modestly well known and the confusion remains.

The supreme example of CCM is, of course, Bill Pullman/Bill Paxton disorder. The subject of jokes on both The Simpsonsand Family Guy, this terrifyingly contagious condition underwent incubation in ideal conditions - similar ages, names and degrees of fame. Despite the advancing years, it still holds the cinema- going public in its unkind grip.

At least Steenburgen/Lahti syndrome seems a little less persistent than before. It was, of course, Mary Steenburgen who appeared in Step Brothersand, I somehow managed to say the right name in my question to Ferrell.

Phew! Let's hope I don't have to interview the director of a Victor Mature film any time soon.

(Or do I mean Tyrone Power?)