[RPG] How to use Passive Perception to have some characters notice parts of the environment

Assuming you can't get a theme, here's my suggestion for a damage-focused crossbow sniper:

====== Created Using Wizards of the Coast D&D Character Builder ======
level 14
Drow, Thief, Dread Fang
Background: Occupation - Criminal (+2 to Stealth)

FINAL ABILITY SCORES
Str 9, Con 12, Dex 24, Int 11, Wis 20, Cha 11.

STARTING ABILITY SCORES
Str 8, Con 11, Dex 18, Int 10, Wis 14, Cha 10.


AC: 29 Fort: 21 Reflex: 30 Will: 25
HP: 89 Surges: 7 Surge Value: 22

TRAINED SKILLS
Stealth +23, Thievery +19, Streetwise +12, Dungeoneering +17, Athletics +11, Acrobatics +19, Nature +17, Perception +17

UNTRAINED SKILLS
Arcana +7, Bluff +7, Diplomacy +7, Endurance +8, Heal +12, History +7, Insight +12, Intimidate +9, Religion +7

FEATS
Level 1: Ruthless Hunter
Level 2: Primal Sharpshooter
Level 4: Two-Fisted Shooter
Level 6: Backstabber
Level 8: Crossbow Expertise
Level 10: Weapon Focus (Crossbow) (retrained to Lasting Frost at Level 11)
Level 11: Primal Eye
Level 12: Wintertouched
Level 14: Silvery Glow

POWERS
Lolthtouched: Cloud of Darkness
Primal Sharpshooter: Grappling Spirits
Thief utility 1: Tactical Trick
Thief utility 1: Escape Artist's Trick
Thief utility 2: Fleeting Ghost
Thief utility 4: Sneak's Trick
Thief utility 6: Chameleon
Thief utility 7: Ambush Trick
Thief utility 10: Counter-Step

ITEMS
Frost Hand Crossbow +3, Feytouched Drowmesh +3, Bracers of the Perfect Shot (paragon tier), Assassin's Cloak +3, Eagle Eye Goggles (heroic tier), Frost Hand Crossbow +2, Potion of Healing (heroic tier)
====== Copy to Clipboard and Press the Import Button on the Summary Tab ======

I calculate DPR with to-hit chances, so my numbers will be much lower than yours.

Ranged basic attack: +23 v. AC, 1d8+26 cold damage Combat advantage from 2 sources: wintertouched/lasting frost with a 1-(28-23)/20=75% chance and hiding based on invisibility or sneak's trick (any source of concealment will do) When you start your turn hidden (for extra juice there, take persistent tail as your level 10 utility), you deal 4d8 as your sneak attack damage.

Tohit: (1-(28-(23+2))/20) = 85% hit rate, with backstabber means you generally won't miss. DPR: .8*(4.5+26+4.5*4+5)+.05*(8+26+8*4+5+3.5*3+.85*(4.5+26+4.5*4+5)=49.14 DPR, all encounter long every encounter. Backstab to be used to turn misses into hits and you're a walking damage machine. AP of course gets you a second verse same as the first, so that's 100 damage on the first round. at 136 average HP for level 14, the slowed enemy (grappling spirits) gets a round to croak for help before you finish them in the second round. No dailies in the class which makes for an incredibly long-enduring character.

Free reroll on stealth checks from your neckslot with a standard action invis from your armor gives you all the stealth you really need. Especially with sneak's trick allowing remarkable stealth in places that would not otherwise be considered venues for sneaking. Average non-trained perception is +10 at 14, so a +23 with a reroll and an II to become re-hidden should provide for sufficient stealth.

For a nova-based assassin... I'll look at the executioner with big, sad, eyes... and go to the Ranger. Their damage simply cannot be beat. I'd personally choose a wind-rider, but that's because I find pure-damage frostcheese excruciatingly boring. The darkstrider gets you your hidden damage boost. Spending feats on skills is usually contraindicated. See here for a goodenough archer that just spams the damage.

If you choose to not go with thief (cunning sneak is slightly less good than sneak's trick, depending) or ranger, then executioner isn't a bad choice. You're trading damage for flavour, but it's very nice flavour. If you MC warlock and take shadow dancer then it's very nice, very very very very mobile flavour. Warlocks also make good assassins MC into assassin :)

Cunning sneak is just not that worth it in actual play. While it helps in theoretical "solo stealth" missions, it's hard being a meat&potatoes assassin. Bravo's a trap. If you're going PH1 rogue/ranger, focus on minor action attacks, multi-attacks, and frostcheese. They'll get you the most damage with the least tradeoffs. Darkstrider becomes better there, because 3+wis damage on multi-attacks very very quickly becomes stupid.

Don't forget to calculate to-hit in your damage potential.

Good luck. Feedback will be incorporated to refine requirements and my suggestions.

When you're just getting started, it is certainly possible to figure out the game from the books and just start running it for your friends. That's how I and others learned to game "back in the day." But there are certainly things that can help.

1. Huh? If you are profoundly and existentially confused about "what is roleplaying", see What is role-playing, and where do I start? and if confused about the general nature of D&D see I'm at a loss with “Dungeons and Dragons.” How does one play it, anyway?

2. READ THE BOOK. The Player's Handbook (PHB), cover to cover. I know you said you've read it, but no really, read it. Especially with 4e D&D they really tried to write to a simpler grade level and have copious examples of anything trying. If you closely engage and read the book, many of those questions will go away (and you'll get less newbie haters when you subsequently go looking for help). Use normal lookups - a word you don't understand? If it's a game term look at the Glossary or index; if it's a normal English word check a dictionary. See also: Playing 4th Ed D&D for the first time, what should I read to avoid holding everyone back?

3. Join a game. This is going to be your #1 route to very, very quickly answering 200 possible basic questions because in one round of combat you'll see people doing all the dice addition, checks, etc. Check with your friends. See Where can I find other RPG players? For D&D, there's a D&D Encounters program where there are demos and games being run in most gaming stores on an ongoing basis. You can join in, or if you just want to watch, you can watch. There's meetups, conventions, any number of places where you can join or observe a real live game.

4. Watch a game. If you are having trouble in meatspace, consider watching one of the many video and/or audio podcasts of what is usually called "actual play." See Where can I find actual play podcasts for RPGs?, search on iTunes, etc. Or you can read play-by-post games, see Where can I find transcripts of actual game sessions?

5. Try the beginner box. If the full game is still too confusing, consider the D&D Starter Set (Red Box). It's fewer rules and more newbie-helpful guidance.

6. Seek help. Needless to say if you know anyone who's played an RPG they can explain it to you better (and I GUARAN-DAMN-TEE you know someone who's played D&D - it's been around since the '70's and have been popular over time, so a parent, a teacher, a friend, someone's done it - ASK AROUND). If you are a brain in a jar, then for super basic "I totally don't know how to do this" questions it's probably quickest to do it in RPG.SE chat. If you want someone to personally take you under their wing and show you the ropes, ask for help there or on a forum, like one of these.

7. Ask questions on RPG.SE. It's not that beginner questions aren't welcome here - there are just some questions so basic that it's much more efficient to have someone in person or chat talk you through "how to add 1d8 + 4" without clogging the site with hundreds of those questions - you'll get answers faster.

Note that as soon as you have played a game, further questions (like about the DMG or MM) can be asked here, that's not too basic, as long as you've read the book first).