Plemons, who is dating Dunst in real life because he's lucky like that, will take on the role of Lenny Shepherd, with Fanning cast as lead character, Esther Greenwood. The mental breakdown suffered by Esther while interning in NYC is said to mirror the authors own bouts with depression. Lenny has kind of a small part, playing the eventual boyfriend of Esther's friend, Doreen.
So this is Plemons doing his lady a solid, but his addition definitely can't hurt. He seems to be on the verge of a breakout, especially if Other People gets the awards season attention it deserves.