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Easter Eggs :
To display a console window press the keyboard keys [Shift] + [Ctrl] + ~.
Next, type "extract" (without the quotes), press [Space], then press [Enter].
A file named "!!!Readme.txt" will be created in the game directory.
The file will give instructions on how to activate various hidden features in the game. Some hidden features include extraction of multimedia files, activation of secrets, and creation of places and animations.
To activate all the Easter Eggs, type "Tippe: SetFlag("egg")" (without the first and last quotes) at the console window.
On some keyboards in other countries, you have to click something other than the ~ key. I'm ot sure what.
When you hit these 3 buttons, the console comes up. In the console window type: ClownShoes and then hit the space bar. Within the set of quotation marks that will appear, type the name of a character and hit the enter key. Examples you could try: "Gabriel", "Grace", "Buthane", "Mosely", etc. Make sure you also try "chicken" and "cat". Try it on Jean while he is cleaning
windows, the Abbe while he is on the roof of the Tour Magdala and Mesmi Day 3, 9PM

Now, there are a bunch of places you can use this "egg" icon for interesting results: Go to the old ruins at Chateau de Blanchefort (top of mountain) and click on the sign pointing to Mt. Cardeau and select the egg icon.

While following Prince jame's men on Day 1, 6-10 PM, click on either man and select the egg icon.

Click on Jean at anytime and select the egg icon. Grace will receive spam on Sidney. :)

Go to the Rennes-le-Baines (the tavern) and click on the barman. Totally fantastic!

Click on the chicken at any time and select the egg icon.

Click on Grace at anytime and select the egg icon.

During Day 2 10-12 PM, go to refrigerator in the hotel kitchen and click on the egg icon.

Click on Emilio during day 1, 10-12 PM while he is sitting in the lobby reading a newspaper, and select the egg icon.

Roger Wilco reference:
When you are able to enter the cleaning room in the hotel, examination of some of the cleaning equipment generates the reference.

To view a cameo by the developer, Jane Jensen:
Find the bookstore window that has the book on the Holy Grail.
Look in the window next to the door to see Jane Jensen, the woman that developed the Gabriel Knight series of games.
In addition, examine the book about the Voodoo Murders (the one Gabriel Knight authored). Gabriel will say "Wow, isn't he some famous author or something?".

Go to the Couiza train station and locate a fallen-over ashtray. Pick up the small note. It says, "Why are you poking around in ashtray sand?" Gabriel says, "Oh ha ha."

Type GK4 in Sidney to get a hint about what gabriel's next adventure will be about. :)