The actor must love his character. If you do not love the character whom you are portraying, then you will somehow comment on that character, negatively. If it’s a comedy, and you are not really playing the character as a real human being, then you will be making fun of the character. There is nothing wrong with this, it’s just a style. This is honestly stock in trade for sketch comedy, improv theater and much of television. But in serious film acting, it is not advisable. You want to be the character without criticizing the character; this is essential. During acting training, I will notice right away if the actor has an aversion to their character. You must be able to justify the character’s words and actions as being appropriate within their world. This is part of the acceptance of the world that the character inhabits.
This does not have to mean that you play only characters that you personally would like or would want to meet. No, you can play your complete opposite, or disowned self—even a person you would detest. But as you do so, you must have the compassion to portray that person honestly. Otherwise, you are not an actor, but an entertainer.