Dannyalcatraz said:UMD? What is that?
Use Magic Device - It's a skill that lets rogues(primarily) use wands, scrolls, and other magical items that they would not normally be able to use.
However, Constantine is not a character that fits well into a system like D&D which tends to be fairly cut and dried in terms of abilities and powers. Most of what he does do is bluff and BS, but he can back it up well enough if need be. However, he is not a knock down drag out raw power combatant. He is a character that depends heavily on essentially "Plot Immunity". He always "Knows" the crucial bit of information, finds the weakness (just not quite in time to save his friend(s)) and only gets his bluff called when he actually isn't bluffing.
If you are going to really capture much of how he operates, you'll really need to work with your DM on your character "knowing" more than the rest of the party (even if it's only the DM telling you that you need to go here and not telling you why) and generally giving the rest of the group the impression that you know a lot more than you actually do.
BTW Constantine the Movie really was pretty decent. About the only area where Keanu really failed was he just can't convey Constantine's charm, but he actually did a credible job and captures the bitterness and sarcastic anger of him quite well. I thought I would hate the film and actually enjoyed it quite a bit.