Sí y No

(Disclaimer: I am not sure the following is only true in Barcelona or only in Cataluña or in entire Spain.)

There seem to be various ways of saying sí (yes) in Spanish:

• Sssss.   In case of a very strong truth, where the speaker is not entirely sure that the person he speaks to is aware of this truth. Also applies in the case where there is a clear disagreement and the speaker really wants to emphasize that the answer is, indeed, sí.
• sÊh. It has to be a really short e and a hardly pronounced h on the end. The s at the beginning is not really there at all. The entire thing hardly makes a sound really. It is used affirmatively, in cases where someone has said something clearly true and the speaker only wants to express his complete agreement.
• Si. All other cases.

There are also two ways to pronounce no (no): 

• T-t-t-t-t (move one finger from left to right and back). Equivalent to ni pensar o ni te ocurre, in other words: no <expletive> way, don’t even think about it, out of the question, absolutely not!
• No. All other cases.