Hostages are NPCs that have been intimidated and are under your control. They are essential for stealth and The Deposit loud. While hostaging is not always needed, it is useful as it can get you intel and other information.
There are two ways to hostage an NPC: Intimidation and Shouting. Intimidation is the most commonly used method, shouting is usually only used during loud. To hostage an NPC you must have a weapon equipped.
You can intimidate NPCs by aiming down sights while looking at them. They will stop and hold up their hands, after which they are your hostage. The detection sound will also play to notify your teammates. You can hold up multiple NPCs if they are within your intimidation range. Once NPCs are fleeing or have their gun drawn they cannot be intimidated.
To shout you simply have to press "F" ("A" on console) with a weapon while looking at NPCs. If there are multiple NPCs in your vicinity, they will all be held up. You can shout at both civilians and guards, but only civilians will be held up. When you shout they will stop and hold up their hands. If you shout again they will lie down onto the ground.
Shouting is not recommended during stealth as it will attract the attention of nearby NPCs, however this can be used to lure guards.
A hostage is very suspicious and they will be detected by any NPC. It is best to bring your hostages to a place where they can not be seen to avoid being detected.
To move a hostage you have to press "F" ("A" on console) while looking at them. They will begin following you if you move away from them. Following hostages have a lack of proper path finding, always taking the direct route to your location, making them often get stuck on doors and tables so you have to lead them appropriately. Once you press "F" on them they will lie down onto the floor. Pressing "F" on them again will make them stand up and follow you. Hostages also move faster during loud.
Certain NPCs can or have to be interrogated. Interrogation is done by holding "G" (Hold "A" on console) while looking at a hostage. This can yield useful intel, a security code or nothing. You can find a list on NPCs that can be interrogated on the respective mission pages.
If a hostage is standing up and the player leaves them alone for too long they escape. Hostages will also attempt to flee if you shoot/knockout several other hostages in their view. Guards will raise the alarm and civilians will run away while calling the police. To avoid this, it is best to knock hostages out after you have interrogated them. Fleeing civilians can not be intimidated, you have to shout at them to stop them.