Witness

The Witness could never dream up the horrors of what she saw. She was with her boyfriend, the Skeptic, when the lights went out and everything flew by in waves. A dark-skinned man in a SWAT jacket tried to usher her towards the door, but she was separated from her lover. Whether bravely or not, she ran back into the darkness to find him, but instead found a pair of bright green eyes staring at her from the darkness. And then the voices... It was a man and a woman speaking at the exact same time, calling for the "Plant". Horrified, she charged to the exit, emerging from the scene of the Final Heist in a surge of panic and terror, before she fell into the hands of waiting officers, where she began telling them what she thought she witnessed behind tears and gasps.

Summary
At the beginning of the Interrogation, the Witness receives the number of active Robbers at the beginning of the game (not including the Attorney). She also receives the names of all players who performed an action on her (as in the Plant checking her card, or the Anarchist swapping her, etc). If the Intimidator tries to intimidate the Witness, he will not be able to intimidate her, but the Witness will not receive the Intimidator's name among the names of everyone acting upon her.

Strategy

 * If you see that 0 Robbers were active at the beginning of the game, this does not mean there are currently no active Robbers. For example, a Wildcard might have taken one from the bottom! You could claim Wildcard to force the real one to out, and if they do, retract as Witness and say there are no Robbers. However, if no other Wildcard claims, it is possible that the real one picked a Robber from the bottom and is planning to claim something else. If there is no Wildcard in the setup, you can immediately say there are no active Robbers.
 * Seeing the number of active Robbers will often help you solve counterclaims between one player and two players, but be wary that if you see that two Robbers were active, this doesn't necessarily mean they're supporting each other's claims. It could also be the one player on the other side of the counterclaim along with someone claiming an entirely different role.