Just use the first word you can think of that expresses your meaning. Don't worry about word choice if it is going to slow you down or interfere with fluency. Don't worry about using the same words over and over. Just be simple and direct. There is nothing at all wrong with just saying "I am glad," "I am sad," "I am surprised," "I agree," "I disagree." You don't need to say it a different way every time. You don't need to say anything fancy. You can convey feelings by the expression on your face and body language.

Also inappropriate word (if you mean swearing) you usually smile if you're joking, or state that you're joking afterwards. It may be different depending on the people and where you are though

Just reply with your response, what you think about what they're saying. Usually it's just the pitch of what your saying and the facial expressions that you portray that will determine your emotion. Like if you laugh while saying something it's usually that they've said something wrong, funny or you don't know what to say. If you are high pitch and say it kind of fast, you're happy or excited. Friend: I saw *someone* today. You: Really :) If you extend your answer. It's almost to say that you disagree or are unsure of what they're saying. Friend: Do you know what the next question is? You: Oh, I~~'m not to sure, sorry. You can pretty much say anything in many ways and they can all mean different things. Try to match your facial expressions with what you intend to say.