There's something about watching someone crack up onstage that has made mental patients such attractive archetypes for actors and audiences alike. For thespians, parts like the civil-services prole in Nikolai Gogol's Diary of a Madman — who loses his sanity after one too many days of mind-numbing drudgery for a monolithic government — provide an opportunity for complete liberation from normalcy, an expressionistic freedom from the regular strictures of real-world acting. Ken Clement made the most of it in this production, arguably the last great show from the now-defunct Mosaic Theatre. He had an occasional assist from actress Betsy Graver, but the production was mostly just Clement, acting through the multiple voices in his head. He was funny, relatable, and ultimately tragic, conveying the play's challenging, staccato script with worn-in ease, slipping into full-on psychosis with disturbing aplomb. That he did all of this in unseemly suspenders and a sand-colored mop for a wig was all the more impressive.