First, you can be prepared. When you know that you are heading into a situation that could be touchy or difficult, where emotions may run high, or where harm could be done if care was not taken, prepare yourself. Be clear about your issues and concerns, and about your boundaries. If you have completely thought through your reasons, reactions and responses to the situation, you will be far better prepared to express yourself clearly. Second, you can be assertive rather than aggressive.
Assertive statements contain no blame and no attack.
Conveying both information and feelings objectively is respectful. Remember, when you are speaking about your feelings you need to own them. When you need to tell the tough truth, take the time to express yourself fully. Give your perspective on the situation, your feelings about the situation and your wants regarding the situation. It is my responsibility to bring this project in on time and it is important to me to do to this.