Let's start with a hypothetical: You've suffered injuries in a car accident that (according to you and several eyewitnesses) was caused by a driver who was on a delivery for an auto parts store. The delivery driver rear-ended you while you sat at a stop light. The police investigation revealed—and the police report filed over the crash concluded—that the driver was momentarily distracted when his phone slipped out of his hand and fell on the floor.
Further investigation discloses that the driver personally carries only the state's minimum required liability car insurance, which won't cover your injuries, lost earnings, and other losses. You've discussed the matter with a car accident lawyer, who has advised you that the best course of action is to sue the delivery driver's employer (the auto parts store) as well as the driver individually. When you bring an action of this type, you are seeking to hold the employer responsible for the employee's conduct under a respondeat superior theory of vicarious liability.
Respondeat superior is a Latin term meaning "let the master answer." In order to prevail on this liability theory in a personal injury case (whether over a car accident or some other kind of mishap), you must prove that the employee was acting within the course of his or her employment at the time of the accident, and was engaged in conduct in furtherance of the employer's business (in addition to proving that the employee was negligent; learn more about negligence in an injury case).
As a general rule, if the employee was not "on the clock" at the time of the accident, or if her conduct was outside the scope of her job duties and responsibilities as defined by the employer, the employer will not be found liable for the employee's wrongdoing, at least not under the doctrine of respondeat superior. A few examples will be helpful to get an understanding of the rule.
First, take the case described above, of the company driver whose negligence causes an accident while he is in the middle of a scheduled work shift, and engaged in the duties of his job for the employer's benefit. In these circumstances, the employer is almost certainly liable for the employee's negligence, and a claim based on respondeat superior against the employer will likely succeed.
Next, take the case of a store security guard who, while investigating an alleged shoplifting incident, commits an unprovoked and violent assault on a store patron. Here, it cannot be reasonably argued that the assault was within the scope of the security guard's defined job duties and responsibilities, or that it was for his employer's benefit. So in this situation, the injured store patron will probably not be successful in pursuing an injury claim against the store, at least not one based on respondeat superior. (The viability of a claim against the guard himself is another matter; learn more about civil assault and battery claims.)
Finally, take the case of a grocery store worker who is shopping at the store on her day off and negligently knocks a glass jar off of a shelf. The broken glass hits and injures another shopper. Even though as part of this employee's job duties she is required to stock and maintain inventory, the fact that the worker was not "on the clock" at the time of this incident means that her employer (the store) cannot be held liable for the other shopper's injuries based on a respondeat superior theory of liability (other theories of fault, including premises liability, may apply).
Why does application of respondeat superior matter in a personal injury case? In most situations, personal injury claimants will want to sue a corporate entity rather than an individual, because these big businesses typically carry much greater amounts of liability insurance coverage. So, it's a common tactic for personal injury lawyers to go after the defendant with the "deepest pockets."