Fisher played the "alternate street speaker voice" that would only appear if the player managed to find and kill the original street speaker.
Carrie Fisher, who unfortunately passed away earlier in the week, is of course best known for her role as Princess Leia. But she did have a number of smaller roles during her acting career, one of which you may have never even noticed. Harvey Smith one of the directors of the original Dishonored, Tweeted out a tribute to Fisher, stating that she was "was gracious, funny and creatively open when we worked with her for a small role in Dishonored."
If you're scratching your head thinking "I certainly don't remember Carrie Fisher in Dishonored" - Fisher's role was a little bit of a secret Easter egg that could only be unlocked in a certain way. She played the "alternate street speaker voice". Fans will no doubt remember the original propaganda spouting street speaker voice was a man, but, during the mission to remove the Lord Regent, you could actually find and kill the street speaker man. If you did, he would be replaced by Fisher, who you can hear in the video above.
It's a small little role that may have been overlooked by many!