John Cusack was once described as having "the innocent Irish face of a very big altar boy." Picture that altar boy with a gun, and you get an idea of the twisted laughs to be had in Grosse Pointe Blank. Cusack is brutally funny as Martin Q. Blank, a professional assassin who has mixed feelings about going home to Grosse Pointe, Mich. -- a swank suburb of Detroit -- for his 10-year high school reunion. "What am I gonna say?" he asks his therapist, Dr. Oatman (a dryly hilarious Alan Arkin). " 'I…