Mainstream offers an alluring cast, but the same cannot be said of its characters. Maya Hawke leads, playing Frankie, a young woman who finds a path to Internet stardom when she starts making videos of Link (Andrew Garfield). Billed as a “charismatic stranger,” Link is more your local loser, who Frankie finds doing Jackass-lite pranks in LA. Later, Johnny Knoxville shows up as a stunt cast, which is delightful, but again the talent isn’t the issue; it’s the stale writing, which makes for a shallow satire. Nat Wolff and Jason Schwartzman round things out as Frankie’s writing partner and Link’s agent, respectively. Together they’re supposed to share a cautionary tale about people trying to preserve their identities within the fast-moving Internet age. The problem is, those identities are never explored beyond nice girl, bad man, good guy, and slightly older bad man, and the film can’t keep up with what it’s trying to comment on.