Star Trek has often had characters struggling to cope with two sides of their personality - Spock with his half-human heritage, Torres with her half-Klingon one - but Jadzia Dax really is two people.

Jadzia is a Trill and Dax is a slug buried within her body. They function as one person with one collective memory but the death of one needn't mean the death of the other - Captain Sisko knew Dax in an earlier body, that of an old man. Not all Trills get these slug symbionts and you have to be highly skilled and trained to have the honour of a slug renting space in your stomach like this.

What Jadzia gets out of it is the shared experience of centuries, meaning that she really is wiser than her years and together with her own abilities is an acutely skilled scientist for Deep Space Nine. She also has the fitness and agility of a young body teamed with Dax's experience which means she's the only person on the station who can beat the Klingon Worf in a fight.

Despite Trills being already known from Star Trek: The Next Generation's episode 'The Host', DS9 producers took a long time to decide to make Jadzia one. Their very first handwritten notes about the show include her name but also the suggestion that she be two separate people unable to get very far apart. Ultimately this is what they got, but by embedding one body inside the other they got some interesting dramatic problems as well as saving on Equity rates.

For a recurring theme in DS9 has been what it's like for Jadzia to have these two melded personalities and, more, what it's like for other people. Sisko still calls her 'old man', for instance, and in an early episode Jadzia was put on trial for murder - a murder allegedly done by Dax and a previous host Trill.

If Sisko hadn't saved the day then we'd have lost one of DS9's most interesting characters - but we nearly didn't get her anyway. Terry Farrell was cast as Jadzia after the show had been filming for a week and didn't walk into Ops until another fortnight had passed either.

Jadzia needs hours in makeup every day while her distinctive pattern of dots on her skin is applied. Those dots only roughly match from day to day - and the make up artists sign their work on her every morning.

Dax, on the other hand, requires no makeup, never forgets lines and to this day has not demanded a separate trailer. It does, though, have it's own internet fan club.

Key Jadzia Dax episodes: Dax, Invasive Procedures, Rejoined