When you walk into a room you suspect the parasite things inhabit. Use the laser and move it around the room. It will glow brighter if one is there.
When in the cabin with Luis (and Ashley), fighting the Ganados coming through the windows, stand by Luis. If you stand near him for long enough, he will start throwing handgun ammo to you.
Before you enter the clocktower section, if you destroy the Novistador nest, a lot of Red / Blue / Green Eye treasures will be dropped.
This trick will summon cannon fodder your way. In the graveyard area, with the church where Ashley is actually held, there is a bell tower connected to the church (to its right). If you ring it, they will come. You can shoot it three times only. The first and second waves of enemies will come from the cave entrance and the third will come from the bridge area.
During the part where Leon and Luis defend the cabin, shoot Luis with your pistol (or any other gun) around 8 or more times. You will get a cut scene where Luis will shoot Leon back, and give you a game over.
When you upgrade any weapon's capacity at a merchant, he will refill the magazine free of charge. This is very helpful with the magnum, or TMP
At any location with chickens, you can sometimes find eggs. If you save nearby, then reset the game and reload your file, then run around in circles in an area with chickens, in about 1-5 minutes, more eggs will be laid! Free health if you ever need it! Best area to do this is the farm.
At various points throughout the game, you will come across different animals such as crows and fish. These animals can be shot and will reveal different items or amounts of money to obtain.
In Professional difficulty, everything is the same except for the fact that enemies are alot tougher to kill.
Near the church in Chapter 1-3, instead of going up to the church, take the path towards the lake. This will trigger a conversation with Hunnigan, where Leon stumbles with words.