Okay, so, first of all, the issue that pisses me off in majority of anime, especially shounen, and a lot of novels, is that dialogues have absolutely no relation to the situation. Like, you have a life and death battle where a second of carelessness can lead to death, and they suddenly start giving each other advice in full sentences. Where's the urgency in that?
Let's not mention the power of friendship and the like, because that's not the topic, but it's definitely closely related.
The second thing I'd like to mention is that to make a good dialogue you need to have both action and expression woven into it. Dialogue without either of those two elements is bland, and you can go only do so much with question and exclamation marks.
Look at this example:
"No! David, don't go down the stairs!" - long, without any expression or action. Bland and unrealistic. Bad, except specific situations.
"No! Don't go!" Adella hissed. - short, you have the action, but there's no specified emotion. Good or bad depending on the situation.
"No! Don't go!" Adella was terrified. - short, you have the emotion, but you don't know what she is doing. Good or bad depending on the situation.
"No! Don't do!" Adella hissed in terror, whipping out her arm as if she could still stop David. - short, urgent, emotional, plus double action to show clearly what's happening. Mostly good, but in this case the description itself might be too long, breaking the immersion.
Now I'm not saying one of those options is the best, but if we're talking about dialogue in general, you can mostly use the same advice. Keep in mind the time, action, and expression, and try to capture every important detail, but not every single detail.
What I described can easily be pushed too far in both directions. The key here would be decorum, so making the dialogue appropriate to the situation. Too long dialogue in quick scenes or too short dialogues during longer talks are equally bad.
As for talking gibberish and metioning things unnecessary to the plot... well, I'd repeat the same advice. If it's appropriate, then you'd certainly make no mistake adding it, but you can just as easily akip it and only mention it being done. For example, if you have a formal meeting of some nobles, you can write the small talk and slowly push in into the actual topic of the meeting, or you can write that at some point during the meal someone mentioned something and go from there.
As for which words fit which character... well, the same advice? Keep it consistent for one character and appropriate for its position. If you have a duchess, you make her speak slowly, deliberately, politely, and not very expressively. If you have a knight, you make it natural and simple, mix in some elements like flattering, flowery language for a well-known knight, short, simple sentences for the knight captain. You can be free with that, because there are always exceptions to the rule, just keep consistent and believable.
