Renders
To get background renders in the game, first you are going to have to finish the game while it is set on private investigator difficulty. You can use any one of the mythos ranks on this setting. For the character renders you'll need to do the same thing.
Art
To get concept art in the game you must also finish the game on the private investigator difficulty setting. You can use any of the mythos ranks here as well. To open up the Craig Mullins gallery, you'll need to finish the entire game on the hardened detective difficulty setting. You can still use any of the mythos ranks.
Total Completion
To achieve one hundred percent completion in the game there are a few things you need to do. You'll have to gather the journal entries for each category. This means you need to collect 13 entries from jack's diary, 18 entries from general evidence and six entries from the mythos tomes. You also have to locate the rifle; you will be able to find it behind one of the reception office doors in the refinery. You also have to save Ruth from the rafters and make sure that you keep enough sailors alive when the boat is under attack. To do this, just don't hide in the ship and you will automatically save enough of them. You also have to finish the whole game in less than three and a half hours and make less than 30 manual saves.
Safe Combinations
You need to locate all necessary clues for each of the safes before you can successfully use the combination on them. For all of these combinations you'll have to turn the dial on the safe clockwise until you hit the first number, then counter-clockwise to the next number. Keep changing the direction for each number until all the numbers in the combination are reached. The clue for the variety store safe can be found in the Waite residence's attic. It is "2, 6, 1, 2." For the church basement safe the clue is to use the postcard on the number tablet. That combination is "3, 1, 5, 7." The clue for the precinct safe is in the cell next to Brian's cell. The combination for that safe is "1, 8, 4, 6." The clue for the Masonic hall safe is in the second floor study. The combination for this safe is "1, 8, 7, 8."