In the most recent episodes, show protagonist David Haller (Dan Stevens) was trying to sort out the mess in his head and discover the true depths of his mutant powers. That crash-course education came in response to the mutant-hunting Division 3 kidnapping David's sister Amy (Katie Aselton), which prompted David to go to some drastic measures to get her back.
In order to access the power to wipe out Division 3 forces and rescue Amy, David let the parasitic entity known as The Devil with Yellow Eyes take over his psyche - often appearing in David's mind under the guise of his former friend Lenny, who is played by Parks and Rec's Aubrey Plaza.
In a recent appearance on Conan, Plaza revealed an interesting behind-the-scenes fact about her casting in the show, telling Conan O'Brien, "In the pilot script, it was written for a middle-aged man," she continued to describe being cast without audition by showrunner Noah Hawley after meeting him, "on crutches at the time because I'd tore my ACL, that has nothing to do with it, but maybe there was something in my behavior."
From the name of the character it'd be easy to see a middle-aged male version of "Lenny" in the storyline - but it wouldn't (in our opinion) have been nearly as effective, as Plaza's comedic energy makes her a standout on the show. Moreover, there's been the reveal that (Mild SPOILER) David's mental recreation of Lenny is actually a false memory, meant to conceal the truth about of his former friend, "Benny" - who is a middle-aged man. That switch wouldn't work nearly as well if the Lenny and Benny actors were both middle-aged guys.