話す and 言う: conversation and utterances
Distinguish having a conversation from saying particular words.
Partner and topic
話す (はなす) means talking or discussing something. 「友達と旅行について話しました」 means that the speaker talked with a friend about travel. と marks the partner and について the topic.
In 「駅員に道を聞いて、友達と話しました」, the person asked for directions differs from the conversation partner. 話す does not make every act of speaking a question.
The particular words
言う (いう) focuses on saying something. 「友達に『ありがとう』と言いました」 means that the speaker said thank you to a friend. It does not establish that they had a long conversation.
「何も言いませんでした」 means that the person said nothing. Without a stated reason, it does not show whether they could not think of anything or deliberately stayed silent.
Language and means of expression
「日本語を話します」 means speaking Japanese, while 「日本語で言います」 means saying something in Japanese. Distinguish the language as the object from the language used to express a message.
「ゆっくり話してください」 asks for slower speech. 「もう一度言ってください」 asks for the words to be said again, so the requested actions differ.
Does 「何と言いましたか」 ask how long the conversation was?
Show the answer and why
No. It asks what was said, not how long the conversation lasted.