Speaking with a Castilian accent... does s always become th?

Speaking with a Castilian accent... does s always become th?
 

So it's kind of a known fact that in castellano "s" sounds like "th"...but it's not always the case, right? Sometimes in castellano it's still correct to pronounce "s" as "s" and not as "th".. how do I know when to pronounce it as "s" and when to pronounce it as "th"?

The letter 's' never sounds like 'th' in Spanish. In Spain, the letters 'c' and 'z' have that pronunciation, in certain areas. The 'z' is pronounced 'th', no matter what vowel follows it. The 'c' is only pronounced 'th' when an 'e' or an 'i' follow it.

It isn't specific enough to call the language castellano, where this pronunciation occurs, since el castellano is spoken in 21 countries. Not all of the people of those countries call their language el castellano, but that is one of its names, el español being the other name people call their language.

Only Spain has the pronunciation you've heard about. So, it would only be appropriate to use it in Spain, in the regions where it is prevalent.

Gotcha! So it's just "z" and "c" in certain occasions... I guess my confusion was grande. Ahora entiendo todo :) Gracias!