Willy never flinches when called upon by the Impossible Missions Force to take part in their plans. He is more than just a strong man. He has unequalled stealth for a man of his size and strength. He is the essence of total calm until there is a storm... then he restores calm.
William "Willy" Armitages main role as an IMF agent was that of "muscle" and a supporting player. In the first three seasons, Willy was brought in on missions to work behind the scenes in mission prep or in minor role-playing such as a waiter at a party or a maintenance man. Often, he would have fewer than ten words of dialogue per episode, and in fact there are two in which he has no lines at all. However, starting in Season Four, his role play, visibility and dialogue as part of missions did expand, including roles that required foreign language skills. While he was not generally used in very complex role play, on at least one occasion his on-the-fly interpersonal skills did save the mission.
Willy's background role meant that he did not get unintentionally captured or trapped as often as other members of the IMF. However, in the Season Six episode "Double Dead," Willy was captured in a mission that was almost blown, and the episode had his recovery as the main focus. In the end, Willy's ability to connect personally with one of his captors would be instrumental in saving his life. In the Season Seven episode "The Deal" Willy was shot after jumping off a boat to avoid capture. While his injuries were serious, he was able to rejoin the team by the end of the episode; however, Willy's disappearance does mark the only time in the entire series that a team member was believed to have been killed in the line of duty.
Even when not out in front on missions, he still played a critical role as missions unfolded, often at a moment's notice. On a regular basis, his split-second timing taking down a sniper or other gunman saved the mission as well as the life of the IMF team member in the crosshairs. In Willy's IMF dossier, it was noted that he had set a world record in weight lifting. His extreme strength was particularly leveraged in several missions, mostly in Season One. However, notably, in Season Five, Willy demonstrated a strength level that bordered on the super-human in tearing a vault door off its hinges to save Barney Collier from certain death in a fire trap. Willy was often called upon to carry or wear extremely heavy objects without visible signs of exertion to betray their weight, often as a way of smuggling teammates in and out of secure locations. He was experienced in hand-to-hand combat, and was often called on to silently disable sentries and policemen with a single blow. He had other skills which were leveraged, but not prominently featured, such as automobile customization and custom construction. He would often custom-build rooms and scenes to make a trapped person believe they were somewhere they were not, such as a rubber room in a mental hospital, a hospital room twelve years in the future, or a holding cell at a slave auction. Also, he was presumably the best marksman on the team, as he would generally be the team member to do any needed pre-planned tasks with a firearm, such as shooting out a tire on a moving vehicle or firing an automatic weapon at someone's feet to get them to surrender. In the episode "Memory," he made reference to having lived in Indiana when he was ten; this is presumably where Willy grew up, as well as the birthplace of actor Peter Lupus.
It is strongly implied over the course of the series that Willy is also in part responsible for mission logistics, particularly the procurement and staging of materials, vehicles etc. in foreign countries. In addition, Willy often acted as support for Barney Collier, particularly when a mission required the use of complicated electronics or required drastic alterations in physical spaces. On many occasions it would be Willy's construction skills that would allow Collier to access the areas required to complete his own tasks.
Willy also ably filled the comparatively less glamorous role of driver for the IMF team. Experienced with a myriad of vehicles, including emergency and construction vehicles, Willy, behind the wheel of a car or panel truck, meeting the rest of the team for their extraction was often the indication of the successful completion of a mission right before the final credits rolled.
Willy Armitage, along with Barney Collier, was one of only two IMF agents that were regulars on the team for the entire seven-season run of the original Mission: Impossible TV series. Like all of the regular IMF agents, he was not used in every mission, but he was a regular character each season even though he was replaced by Sam Elliot's character in the opening credits of some Season Five episodes. Willy was the only non-smoker out of all the regular IMF team members for the first three seasons.