If memory serves the back story on Tom Cruise's character Vincent included that he grew up in a lower class neighborhood in Gary, Indiana and he was an ex Navy Seal (or Green Beret). None of this is actually mentioned in the film. It is alluded to when Vincent is asked how long he has been doing this.

"For the private sector? Six years."

Collateral is flawed IMHO, but on the whole works rather well, one of the better films of the past year. Cruise's performance is terrific. Every good actor needs to portray an out and out bad buy at least once in his career (e.g., Alan Arkin in Wait Until Dark; Henry Fonda in Once Upon a Time in the West)