Her character on Doctor Who, Sarah Jane Smith, whom she originally portrayed during the 1970s under producer Barry Letts, for a mere three years (1973-1976), made arguably the most fundamental shift in dynamic seen on the show. Who had seen smart female characters and companions before Sarah Jane, most notably the elfin genius Zoe, who travelled in Patrick Troughton's TARDIS, but where Zoe had been a girl of immense scientific abilities, Sarah Jane was the first female character who was, properly, a fully-rounded woman. Clever, inquisitive, and no-nonsense, she redefined the role of women in Who, and showed that it was possible for women to exist within Who who were not required simply to scream and run.
She was also a tremendous anchor for the show for the audience, suggesting that you too, the watching kids, could be brave, investigative, and sensitive. Sarah Jane was a reporter, a job treated with distain by Jon Pertwee's Doctor, but she embodied the best elements of that role. She questioned everyone with the same determined style, even the Doctor, and through her you could learn that it was okay to question authority figures too.
It also proved that you, the audience, could hold your own against the genius of the Doctor, or the military authority of the Brigadier, if you asked the right questions and never gave up. Sarah Jane Smith did this, and it was important. It's also worth noting that she wasn't sexualised in the style of Jon Pertwee's earlier companion Jo Grant, or Tom Baker's later companion Leela. This isn't to suggest that the show shouldn't be sexy, and certainly Who has exploited this, from Peter Davison and Nicola Bryant to Matt Smith and Karen Gillan, but Sarah Jane offered probably the most rounded, human, companion that ever stepped foot in the TARDIS during Who's original run.
Sarah Jane's return during Who's reboot, in the David Tennant-era 2006 story School Reunion, was for Sladen an unexpected late success, spinning off into a children's BBC show, The Sarah Jane Adventures. Ironically, a 1981 special, K-9 & Company, starring Sladen and the Doctor's robotic dog K-9, was intended to lead to a spin-off show itself. That Sladen did eventually gain her own show as Sarah Jane more than twenty-five years later is testament to the continuation of the audience's affection for her character, her own love of Who, and the unwitting influence that she had had on the writing and production staff of the revived Who when they had themselves been watching Doctor Who on Saturday nights during their childhood. Every female Doctor Who companion seen in the revival - Rose Tyler, Martha, Donna, and Amy Pond, has a little bit of Sarah Jane in them. Yet, as stated before, this is for a good reason. Sarah Jane was the most human, the most real, of all the Doctor's companions, and so rightly became the template for How To Do It Right. This is a great testament to the talents of Liz Sladen, that in three years she redefined the role of the companion into a driving force, rather than merely a reactive one.
An obviously generous, sensitive, and private soul, through interviews you could tell the huge affection she held for Doctor Who, and for Sarah Jane. The fact that, as Tom Baker has said, no-one knew that she was suffering from cancer makes the loss all the more shocking, and all the more affecting.
Forty years' worth of children have grown up admiring Sarah Jane Smith. Indeed, at the age of 8 or 9, in the mid-90s wilderness, I first watched Who on VHS tapes from the local library, and encountered a whole universe of adventure. But the person who made eight-year-old me believe, if the TARDIS were to materialise on my doorstep to sweep me away, that I would do all right, was Sarah Jane.
So for all the small children who felt the same, Liz Sladen, thank you.