Speech.
You can only initiate speech against a neutral or friendly character. Speech is an important skill to increase. Speech is use in information gathering, gaining additional information from that you normally do and to con or bluff you way out of a combat situation or to gain benefits from other Non-Player Characters. Game Master can also use this skill to see if the player can gain additional information or even access to "other stuff" or the back room stuff from a merchant or trader. If the player are hopelessly hopeless in his actual speech skill and that he will get himself kill, Game Master can hint or ask the player to roll his speech skill to keep him alive, unless, he deserve to die. Speech skill is also sometimes used during bartering, for example, if you wish to gain access to his private goodie stuff or if the town are racist, and you wish to gain access to any facilities (doctors, inns or trading), depending on the way the player express his/her speech is convincing or not. It depends on the Game Master wishes to allow you to gain access to them. If so, the speech skill check is needed. The difficulties are varies. Remember, Fallout is a harsh world! After adding the difficulty penalty, and that if your skills reach 0% or below it then he or she could not use them.