Speech is a skill in The Elder Scrolls V: Skyrim and is one of the six skills that falls under The Thief play-style. This skill is a combination of both Speechcraft and Mercantile skills of previous Elder Scrolls and replaces them. Speech may also help with quests by unlocking special dialogue interactions, including Intimidate, Mock, Brawl and more.
Speech directly affects the prices paid when buying from and selling to merchants, even without the assorted perks. At 0 speech, merchants sell items for 3 times their base price, and buy for a third of their base price. At 100 speech, merchants will sell items for twice their base price and buy them for half as much. Alternatively, Speech can be used to persuade people into doing things or divulging crucial information. Bribes made are also governed by Speech.
Can invest 500 gold with a shopkeeper to increase their available gold permanently (see Note)
Intimidation
Speech 70, Persuasion
Intimidation is twice as successful
Fence
Speech 90, Investor
Can barter stolen goods with any merchant you have invested in
Master Trader
Speech 100, Fence
Every merchant in the world gains 1000 gold for bartering
†The way Investor seems to work is that the 500 gold ends up in the shopkeeper's personal inventory rather than the store inventory, also, it seems like any gold in the shopkeeper's personal inventory is added to the store gold when calculating how much they can buy. This means two things:
can be pickpocketed from them. This means the store will not have 500 when their inventory refreshes. Stealing from them does not lower their disposition.
can be reverse-pickpocketed onto them to increase the investment effect. Also, placing gold is much easier than taking it. This can create shopkeepers with a very high amount of gold.
The speech skill increase depends on the base value of the item traded or the difficulty of the bribe or persuasion. When selling items, it is not actually required to receive any gold, only that the transaction takes place. The skill increase only occurs once per transaction. In order to optimize leveling, expensive items in groups of 6 or more should be sold individually, rather than all at once.
Jumping from Bard's Leap Summit at Lost Valley Redoubt south of Roriksted and surviving raises Speech when the Bard's Ghost at the bottom is spoken to again.
At the Black-Briar Meadery in Riften, the Dragonborn can, through a Persuasion check, enter an infinite dialogue loop with Ungrien regarding Maven Black-Briar. Once at the Meadery, speak to Ungrien and ask him to tell you what he thinks about Maven Black-Briar. Then, choose the Persuade option "Come on, get the truth off your chest". This is no longer possible with the new update. It will make the dialogue disappaer after one try. Fixed as of Patch 1.9.
Members of the Thieves Guild have the option to bribe guards when minor crimes are committed. Repeatedly committing small crimes, such as trespassing or pickpocketing, and bribing guards to look the other way can help level Speech quickly. spent bribing a guard can be pickpocketed back with a high enough pickpocket skill.