Viewing All Models:
Create a shortcut to MaxPayne.exe primarily located in C:/Program Files/Rockstar Games/Max Payne (depending on your distribution). Right click on the shortcut and click properties. In the Target field, type this in if your installation is in it's primary directory: "C:\Program Files\Rockstar Games\Max Payne\MaxPayne.exe" -developerkeys
Next, double-click the shortcut and go into the game. Load it up and use the Page Up and Page Down buttons on your keyboard to switch through the different character models.
The Complete Hack
Create a shortcut to MaxPayne.exe primarily located in C:/Program Files/Rockstar Games/Max Payne (again, depending on your distribution). Right click on the shortcut and click properties. In the Target field, type this in if your installation is in it's primary directory: "C:\Program Files\Rockstar Games\Max Payne\MaxPayne.exe" -developer
Next, double-click the shortcut and go into the game. Load it up and hit the F12 key to open the developers console. Here are a list of commands you can use to cheat:
Here are a list of other commands that are mostly self explanatory.
Taking screenshots in game is actually possible without a third party program or using the Print Screen key. Just create a shortcut as I told you how to above and in the Target field type: "C:\Program Files\Rockstar Games\Max Payne\MaxPayne.exe" -screenshot
Please note that you can use multiple command line parameters. For example: "C:\Program Files\Rockstar Games\Max Payne\MaxPayne.exe" -developerkeys -developer -screenshot