For people like me who played the HD Collection on PlayStation 3, I was incredibly confused to have to adjust to the lack of pressure-sensitive controls. I didn't see anyone talking about this yet, so I'll be the first:

To aim, hold the right bumper (RB) to go into first person mode and hold down the Fire Button (X) to fire. This proves to be incredibly frustrating when you just want to line up your shot and end up accidentally wasting a bullet. This is even worse when holding a fully-automatic weapon like an AK-47, which shoots as soon as you press the Fire Button.

Bullets are very precious in the jungle, so here's a little tip to help in your sneaking mission.

Click the left stick to have Snake hold and aim his weapon. It's an easy way to hold people up without wasting a bullet, or fiddling with the controls to cancel your aim.

For interrogation, you can also click the left stick to threaten them with your knife, while the Action Button (Y) slits their neck.

Hope this helps!