This seems a little bit of a broad question. Are you asking for just in general or for a particular situation?
Appropriate would be respectful of the child. They are smarter than most give them credit for. I find you'll get the best response if you get down on their level. Bend down or better yet squat down when talking to them. Depending on the age of the child will change a little how you talk to them. you wouldn't want to talk to an 8 year old like you would a 2 year old. With the very young keep it simple. Bring me the toy. Get your cup. No, that will hurt, No Touch. Walk Inside, Run Outside. With an older child you can hold full conversations with and they will understand. Of course using proper words in their presence and good respectful manners in front of them. Children will copy and imitate adult behaviors and words, so make sure you are setting a good example. And watch the tone of voice you use with children. They will pick up on your body language and tone of voice quickly.
Inappropriate behavior of course being touching in private places, vocabulary that should not be used around children as well as adult topics. And a big one is just the lack of supervision. Make sure especially if they are young you are staying observant to what is going on. A lot of is is common sense, and most of these things most people will tend to do naturally when around a child, but just be mindful of your behavior when near them.