Reno is not Hamlet, or Lady Macbeth. And, in 1934 they famously threw together both the book and characters (In a nightclub, "Isn't that Aimee Semple McPherson? Jot down, "Nightclub evangelist singer,") as the basic structure upon which to hang those immortal songs.
Still, Foster does more simply sublime acting within a song while singing "I Get a Kick Out of You" or or in the affectionate playfulness of "You're the Top" than most Broadway actors do in an entire straight play.