Gabriel's advice is sound as always
If you're playing an assassin, the main thing you will be wanting to do (if you're interested in role-playing the character properly) is assassinations - or backstabbing. So to be a realistic assassin, you should develop your stealth skills really well, letting the other thieving skills become secondary.
You should still spend some points on detect traps, as it will save your hide, but I wouldn't put anything into the other skills until your stealth (hide in shadows or move silently, I forget which) is really high, way over 100. Pick Pockets should be at the bottom of your list for development. If you want be good at that, play a bard, it comes naturally to them!
I am of course assuming that you picked assassin because you wanted to assassinate - if not, please feel free to ignore my advice!!!
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Time is an illusion. Lunchtime doubly so.