I remember one anecdote with our gaming group whit this theme. It was remembered as "the 24 incident" 
Friend wanted to intimidate an NPC and w/o saying anything went on with intimidate roll and got a 24. When the DM asked him what do you say to him, he looked confused for a sec then looked at his d20 and his Intimidate bonus and said "24". We had a good laugh and for few sessions when we wanted to intimidate NPC we just roleplayed that we come to them and yell from the top of our lungs "Twentyyyyy FOAAAAAR!!"
On more serious note: I think that the PC MUST! say what he's character is saying to the NPC and then roll the intimidate/diplomacy check and DM grants a bonus/penalty to the roll depending on what has been said.
Any one can say: I will kill you if you don't give me the money, but it will be more convinsable from Tony Soprano than the street thug.

Friend wanted to intimidate an NPC and w/o saying anything went on with intimidate roll and got a 24. When the DM asked him what do you say to him, he looked confused for a sec then looked at his d20 and his Intimidate bonus and said "24". We had a good laugh and for few sessions when we wanted to intimidate NPC we just roleplayed that we come to them and yell from the top of our lungs "Twentyyyyy FOAAAAAR!!"
On more serious note: I think that the PC MUST! say what he's character is saying to the NPC and then roll the intimidate/diplomacy check and DM grants a bonus/penalty to the roll depending on what has been said.
Any one can say: I will kill you if you don't give me the money, but it will be more convinsable from Tony Soprano than the street thug.