Robert O'Ryan was taken into custody last March (09) after he attempted to break onto the Los Angeles set of Dancing With The Stars, where Johnson was a competitor.
Authorities later found duct tape, a shotgun, a handgun, a knife and a bulletproof vest in his car, and he was charged with felony stalking, felony commercial burglary and two misdemeanour counts of carrying a loaded firearm in his car.
He has pleaded not guilty by reason of insanity to the two felonies and is currently on trial at Los Angeles Superior Court.
Johnson, 18, took to the witness stand on Thursday (10Jun10) to describe how the stalking threat forced her to scrap promotional appearances and change cars and even consider leaving the TV competition so she could move away.
She told the court, "To see that somebody had that much obsession, it really made me scared.
"We had thought about leaving the show and moving home if it was necessary. We had kind of changed everything. I was told I couldn't go shopping or basically (go) to public places."
Johnson went on to win Dancing With the Stars and become the show's youngest ever winner.
An attorney for O'Ryan, 36, maintains her client was not a stalker but merely had a "crush" on Johnson.
The case continues.