negative effects for a bad diplomacy check?

[Kyramus]

Most people in my game don't use charisma checks, so I just put it down as a diplomacy check untrained or not.

when an Npc and a PC negotiate, it's opposed roll, if the pc's roll high it can shift the NPC's attitude towards friendly, and if the PC rolls low the NPC might not budge from his position or turn hostile.

works great so far.

 

[Ridley's Cohort]

the problem that arises when role-playing takes over rolls,in our group, is that it tends to penalize those within the group that can't act well or good. plus, i think the other players in the group would resent having to make a roll to determine how well their character got along with others. they would rather "role-play" than rely on a "roll".

As already pointed out, acting out of character is bad roleplaying. If you want to take advantage of your good acting skills, you should put at least a 12 in Charisma and spend a skill point now and then on your social skills. From a rules perspective, it is fundamentally no different from reading the Monster Manual while at the gaming table.

BTW, a good DM won't necessarily bother to have you roll the dice, but he will still use your social skill ratings to help decide how what you say is interpreted.