A pneumatic linear actuator is presented and evaluated. Designed to operate in demanding environments such as MRI, it is developed to be used with two motion control modes: 1) a step-by-step mode with tooth-based gripping to ensure precision, 2) a continuous mode available locally for fine positioning. The actuator can also be disengaged to enable direct handling by an operator, for example for comanipulation. The design is presented. A prototype, developed in the medical context, is implemented and characterized. A specific step-by-step control sequence is then elaborated based on its characterization. Testing of the dual-mode actuation is finally described. The complementarity between the two motion modes and possible adaptations of the original design are discussed.