The former Royal Ballet star, who will feature on the final three episodes, said that she would be "diplomatic" but honest.
Bussell told Radio Times: "I'm glad I'm coming in at the end of the competition when the dancers are less nervous about performing, because you can be much more critical at that stage.
"So I will not be shy about criticising them and saying, 'That didn't work. It hasn't improved. You need to do a lot more work before you come back next week'."
BBC One bosses confirmed in July that Bussell and Alesha Dixon would be replacing Arlene Phillips on the judging panel.