quality:correction, stylistic:tone
Acting style and screen persona

He had the ability to talk naturally. He knew that in conversations people do often interrupt one another and it's not always so easy to get a thought out. It took a little time for the sound men to get used to him, but he had an enormous impact. And then, some years later, Marlon came out and did the same thing all over again—but what people forget is that Jimmy did it first.

—Cary Grant on Stewart's acting technique.

According to biographer Scott Eyman, Stewart was an instinctive actor. He was natural and at ease in front of the camera, despite his shy off-screen personality. In line with his natural and conversational acting style, Stewart's co-stars found him easy to work with, as he was willing to improvise around any situation that arose while filming. Later in his career, Stewart began to resent his reputation of having a "natural" acting technique. He asserted that there was not anything natural about standing on a sound stage in front of lights and cameras while acting out a scene.

Stewart had established early in his career that he was proficient at communicating personality and character nuances through his performances alone. He used an "inside-out" acting technique, preferring to represent the character without accents, makeup, and props. Additionally, he tended to act with his body, not only with his voice and face; for example, in Harvey, Stewart portrays the main character's age and loneliness by slightly hunching down. He was also known for his pauses that had the ability to hold the audience's attention. Film critic Geoffrey O'Brien related that Stewart's "stammering pauses" created anxious space for the audience, leaving them in anticipation for the scene which Stewart took his time leading up to.

Stewart himself claimed to dislike his earlier film performances, saying he was "all hands and feet", adding that he "didn't seem to know what to do with either". He mentioned that even though he did not always like his performances, he would not get discouraged. He said, "But I always tried, and if the script wasn't too good, well, then, I just tried a little bit harder. I hope, though, not so hard that it shows." Former co-star Kim Novak stated of his acting style that for emotional scenes, he would access emotions deep inside of him and would take time to wind down after the scene ended. He could not turn it off immediately after the director yelled cut.

Stewart was particularly adept at performing vulnerable scenes with women. Jack Lemmon suggested that Stewart's talent for performing with women was that he was able to allow the audience to see the respect and gentility he felt toward the women through his eyes. He showed that his characters needed them as much as their characters needed him. In connection to Stewart's screen persona with women, Peter Bradshaw said The Philadelphia Story is "a film every school pupil should see" due to Stewart's character's clear explanation of sexual consent after being accused of taking advantage of the main female character.

Stewart's screen persona was that of an "everyman", an ordinary man placed in extraordinary circumstances. Audiences could identify with him, in contrast to other Hollywood leading men of the time, such as Cary Grant, who represented what the audience wanted to become. Stewart's screen persona has been compared to those of Gary Cooper and Tom Hanks. Eyman suggested that Stewart could portray several different characters: "the brother, the sweetheart, the nice guy next door with a bias toward doing the right thing: always decent but never a pushover". In Stewart's early career, Louella Parsons described his "boyish appeal" and "ability to win audience sympathy" as the reasons for his success as an actor; Stewart's performances appealed to both young and old audiences. According to film scholar Dennis Bingham, Stewart's essential persona was "a small-town friendly neighbor, with a gentle face and voice and a slim body that is at once graceful and awkward". Unlike many actors who developed their on-screen persona over time, Stewart's on-screen persona was recognizable as early as Art Trouble (1934), his uncredited debut film role, where Stewart was relaxed and comfortable on screen. He portrayed this persona most strongly in the 1940s, but maintained a classic everyman persona throughout his career.

Film scholar Dennis Bingham wrote that Stewart was "both a 'personality' star and a chameleon" who evoked both masculine and feminine qualities. Consequently, it was difficult for filmmakers to sell Stewart as the stereotypical leading man, and thus he "became a star in films that capitalized on his sexual ambivalence". Stewart's asexual persona as a leading man was unusual for the time period for an actor who was not mainly a comedian. However, during his career "Stewart the furthest extremes of American masculinity, from Reaganite militarist patriotism to Hitchcockian perversity".

According to Roger Ebert, Stewart's pre–World War II characters were usually likable, but in postwar years directors chose to cast Stewart in darker roles, such as Jeffries in Rear Window. Ebert put this into contemporary perspective by asking, "What would it feel like to see in a bizarre and twisted light?", explaining that it is jarring to see a beloved everyman persona such as Stewart in dark roles. Furthermore, Jonathan Rosenbaum explained that since audiences were more interested in Stewart's "star persona" and "aura" than his characters, "this makes it more striking when Anthony Mann and Alfred Hitchcock periodically explore the neurotic and obsessive aspects of Stewart's persona to play against his all-American innocence and earnestness."

Film scholar John Belton argued that rather than playing characters in his films, Stewart often played his own screen persona. He had difficulty playing famous historical personages because his persona could not accommodate the historical character. Belton explained that "James Stewart is more James Stewart than Glenn Miller in The Glenn Miller Story (1954) or Charles Lindbergh in The Spirit of St. Louis (1957)." Moreover, Jonathan Rosenbaum continued that Stewart's "pre-existing life-size persona" in Winchester '73 "helped to shape and determine the impact of in ". On the other hand, Stewart has been described as a character actor who went through several distinct career phases. According to film scholar Amy Lawrence, the main elements of Stewart's persona, "a propensity for physical and spiritual suffering, lingering fears of inadequacy", were established by Frank Capra in the 1930s and were enhanced through his later work with Hitchcock and Mann. John Belton explained that "James Stewart evolves from the naive, small-town, populist hero of Frank Capra's 1930s comedies to the bitter, anxiety-ridden, vengeance-obsessed cowboy in Anthony Mann's 1950s Westerns and the disturbed voyeur and sexual fetishist in Alfred Hitchcock's 1950s suspense thrillers." During his postwar career, Stewart usually avoided appearing in comedies, Harvey and Take Her, She's Mine being exceptions. He played many different types of characters, including manipulative, cynical, obsessive, or crazy characters. Stewart found that acting allowed him to express the fear and anxiety that he could not express during the war; his post-war performances were received well by audiences because they could still see the innocent, pre-war Stewart underneath his dark roles. According to Andrew Sarris, Stewart was "the most complete actor-personality in the American cinema".