The growing demand for compact and compliant mobility assistive devices has driven interest in low-profile actuating technologies. With the increasing mobility needs of an aging population, such devices could meet this growing market if they can provide low power capabilities, high strength, and compatibility with standard industrial fabrication processes. As a result, researchers have been investigating smart materials, such as carbon nanotube (CNT) and their higher-order structures, as potential components for soft actuator systems. However, reported works using this material within actuators have remained limited due to the material's prohibitive cost and fabrication complexity. Furthermore, presented actuator designs are difficult to compare due to custom fabrication procedures and inconsistent characterizations. The recent availability of commercial higher-order CNT products and the superior material consistency they provide present an opportunity to comprehensively analyze these materials in actuators without the challenges faced in previous work. This thesis addressed this opportunity by evaluating a stiffness-variable actuator design leveraging a commercially available CNT fibre. The evaluation focused on the effects on the mechanical and electrical properties in addition to its electrothermal and electromechanical responses when changing selected actuator design and operational parameters. The findings highlight the importance of optimal coating and embedded pre-stretch to achieve optimal contractile stress and contractile strain performance, while increased fibre diameter diminishes these properties. Furthermore, the usage of commercial CNT yarn ensured consistent mechanical and electrical properties during the fabrication and testing of actuator prototypes. This in-depth understanding of this actuator design's strengths, weaknesses, and the influence of selected operational and design parameters on performance establishes a foundation for future CNT-based actuator research within a repeatable framework.