The difference in difficulty between easy, normal and hard is minimal. Noticeable is only the number of hits it takes to down an enemy. The investigation itself is not influenced by the difficulty.
Once you have beaten a chapter you can replay that chapter from the main menu. Simply use "choose chapter" instead of "continuel". All progress made during a replay is saved into your savefile. This feature can be used to do sidequests which you might have missed.
If you consider this cheating then there is another way to do all quests. Simply go to the graveyard and sleep untill the conditions/time are right. After sleeping on the graveyard make sure to eat from the endless supply of tomatoes.
At first the gameplay seems to steer you toward following the main questline. If you deviate from the main quest then the NPC react somewhat irritated towards you. For example they remark about your lack of teamwork.
My advice is to simply ignore them/this and start doing sidequests. A lot of trading cards can be collected by doing sidequests. Also the rewards from these sidequests will make your investigation easier.
I normally go for the following rewards:
- Wrench (infinite). Available Chapter 2
- Weapon Bag. Available Chapter 2
- 10 mm Submachine Gun (infinite). Available Chapter 6
- Magnum (infinite). Available Chapter 8
- Wesley Special (infinite). Available Chapter 8
- Big Bag. Available Chapter 13
Wrench: this melee weapon will enable you to navigate the normal shades with ease. This weapon is strong and fast. It stuns them before they can attack you. Bossis and wall-crawlers require a ranged attack.
Weapon Bag: enables you to carry more weapons. If you choose not to get this item then you need to carefully manage the weapons in your inventory.
10 mm Submachine Gun: I like to use this weapon untill I can get my hands on the Magnum. The major advantage of this weapon is the enormous magazine with 360 rounds. This weapon quickly kills the shades. In addition it can be used to stun a group of approaching shades. I highly recommend this weapon to finish the final boss fight. Use this weapon when you climb the final boss and have to hit the doll in his chest-pocket.
Magnum: This is my favourite weapon. 2 shots will kill a shade on hard. 1 shot is sufficient on easy. Disadvantage is the small magazine with 4 rounds. Reloading has to become second nature. I use this weapon on shades, wallcrawlers and bosses.
Wesley Special: This is the flamthrouwer in the game. Some people will use this for wallcrawlers. But I prefer to use the magnum against the wallcrawlers. Never use this on normal shades for they will continue to suicide-run towards you. Basically I use this weapon only at one point in the game. I use it in the other-world art gallery to get the green, red and ripe apple from the tree.
Big Bag: this enables you to carry more items. I usually get this because the quest to get it is really simple and short.
The above weapons are not the most powerful in the game. But they are early available and they suffice to navigate the whole game. During the last few chapters of the game some really powerful weapons become available. For melee I like the grasscutter and the infinite RPG is also a nice addition.
The radio is almost a must have for the game. It allows you to fast travel to any areas you have been to before.
The radio can be obtained by picking the "nameless flower" in the rain on the road to the lumbermill (check the map for the flower symbol). Wait outside George Woodman's house until it is 22:00 then knock on his door to start the side quest. Finish the sidequest by giving him the flower. The radio doesn't work in replays.