In the series Shameless.US, S06E10, an actress utters "leave immediately, sooner if possible". Is this idiomatic? I'd like to know why sooner refers to.
It isn't quite idiomatic, although there are plenty of similar expressions.
It's just humor.
Clearly one cannot leave sooner than immediately.
Some experts assert that Franz Liszt's legacy includes a piano score with the author's note at the top of each page, and those notes progress as follows:
Page 1: "Play very fast." Page 2: "Play faster." Page 3: "Play even faster." Page 4: "Play as fast as possible." Page 5: "Play faster still."