Movie production does not happen in a vacuum. Especially with big budget movies like Marvel's Captain America: Civil War, studios rely heavily on test screenings to determine if audiences are going to like a movie or if rewrites and reshoots will be needed to make the movie more palatable to general audiences.
In some cases, this has led to extensive delays as studios desperately try to rework the movie into something that will not be a complete box office disaster. That is no guarantee of a solid final product, though, so the ideal scenario is when a movie is a hit with test audiences right out of the gate.
According to Birth. Movies. Death., Marvel has found itself in that best case scenario with test screenings of Civil War, which "has been testing exceptionally well" with audiences. This is reportedly confirmed by five different sources, which is a good sign for the veracity of the claims.
This means that the movie will be able to make it out the door with minimal changes, although the cast and crew "are back on set right now, picking up stuff for the upcoming Phase Three opener." This is likely more to add some scenes for continuity with the MCU at large rather than to fix problems with existing footage, but it could also be that they found that minor tweaks were necessary based on feedback from the test screenings. This happened with Avengers: Age of Ultron, when Loki was cut after test audiences were confused about his role in Ultron's plan for world destruction.
Either way, the positive feedback from test screenings is a good sign for Civil War. Bad publicity about poor test screenings can not only scare audiences away from a movie, it can also portend that the final product won't be very good. The more a movie's final cut is determined by committee, the more it is likely to be unsatisfying. Thankfully the filmmakers Marvel has brought on tend to have a pretty good handle on what they are doing, despite (or perhaps because of) the reportedly "no rules" attitude of the studio.
While positive test screenings alone are not proof positive that a movie is going to be good, the Russo brothers turned in one of the best MCU movies so far with Captain America: Winter Soldier, so there's little reason to think they won't do the same with Civil War. As long as they are able to juggle the massive amount of characters who will be appearing in the movie (although not as massive as the upcoming Avengers movies), they may be poised to top themselves when Captain America returns to the big screen this May.
Captain America: Civil War will release on May 6, 2016, followed by Doctor Strange – November 4, 2016; Guardians of the Galaxy 2 – May 5, 2017; Spider-Man – July 7, 2017; Thor: Ragnarok – November 3, 2017; Black Panther – February 16, 2018; The Avengers: Infinity War Part 1 – May 4, 2018; Ant-Man and the Wasp – July 6, 2018; Captain Marvel – March 8, 2019; The Avengers: Infinity War Part 2 – May 3, 2019; Inhumans – July 12, 2019; and as-yet untitled Marvel movies on May 1, July 10 and November 6, 2020.
Source: Birth. Movies. Death.