Karmic acts are generally divided into three categories in Buddhist writings, these being mental, verbal, and physical acts. To illustrate, if I form hateful thoughts about someone I meet, that is a mental act. If I tell the person hateful things to express those hateful thoughts, that is a verbal act. If I strike that person with a stick or fist, that is a physical act. Each of these types of acts have different consequences and gravities.
Mental acts tend to function to condition our attitudes and opinions.
Verbal acts tend to condition our relationships with others, and usually will also reinforce mental conditioning. Physical acts are often the most powerful, because they can go so far as taking or perserving life. They have results that condition our physical environment. Generally speaking, though, karmic acts tend to have their seeds in mental acts that, if nutured, blossom into verbal and physical acts. To speak and behave compassionately, on a consistent basis at least, one needs to begin with compassionate mental acts.