"One-on-Ones" are a critically important piece of the anti-union campaign. Supervisors and sometimes managers will call employees in for face-to-face "discussions" about the union. These are not voluntary nor are they casual. If it is on paid time, employees have no choice but to go if the owner or supervisor insists. Remember, for the amount of hours they pay you, they essentially own you and they can require you to go to these meetings. 

In a one-on-one, you are entirely alone with your supervisor! That can be pretty scary and intimidating! And, that's the point!!! 

The company owner or supervisor have an agenda to follow. It may be constructed around one of the boss's letters or be more conversational. In any event, the owner or supervisor has the employee alone in a room with no one there to support or defend the employee or to counter what the owner or supervisor says. There is no equal time for the union side. Most of the time they do not target the strong union supporters but the more vulnerable employee who they believe can be moved to their side. 

The more intense one-on-ones are more frequent later in the campaign, especially in the 4 to 6 weeks before an election.