How do you interrogate guards on Metal Gear Solid 3

There is no definite way to 'Interrogate' a guard, but the main two ways are:

1. Grappel/take hostage a guard and hold a gun to his head (to grapple a guard, see game hand book)

2. Get close to a guard and aim a gun at them, the larger the gun the more likely it is to work. Your character will then shout either 'Hands up' or 'Freeze'.
You then need to point the gun at either their head or crotch to question them.