It seems that in most cases, the two are interchangeable. For example: Is it 說 /说 / shuō or 講 / 讲 / jiǎng for "to speak a language"?
Apart from fixed phrases, when would you use one rather than the other? When is only one suitable? Are there regional differences to this preference and what are they?
Generally, both are the same meaning of "speak". But still have a little difference:
"讲"--(as a verb) to speak, to give a speech, to say something in a logical way
"说"--(as a verb) to say, to talk
"说" and "讲" can be tricky, when both 说 and 讲 are interchangeable, 讲 feels a little more colloquial whereas 说 is usually used on formal occasions.
Example: 说谎 = To tell a lie
说明 = to explain (literally, say clear).
说不定 = perhaps/maybe.
Example when both are interchangeable
说话／讲话 = speak/talk
说故事／讲故事 = tell a story
你说什么？／你讲什么？ = What are you saying?
Sometimes 说 = 讲, in Chinese, one word may have a lot of different meaning, and a lot of words may have same meaning. In normal, 说 like we daily use to speak, 讲 like official statement which more formal.
