Actors are obviously important to a movie. To be a great film, you pretty safely also need great acting, with rare exceptions.
Great acting is a small part of the pie though.
Southpaw is the newest example of this scenario, using a strong performance by Jake Gyllenhaal, who has been on fire of late, and plopping it into a pile of worn out tropes, bad dialogue and the like. Unless you’re Queen Swinton, expecting a quality movie from the cast instead of the director is a dangerous game. My full thoughts on why Southpaw fails can be found on Examiner.