Assume a self-assured, dominant body posture. For this, your shoulders should be slightly back, chin up. When you are speaking to the other person, make direct eye contact.
Speak clearly. Bossy people want their wishes heard and followed. A bit of over-enunciation is also appropriate if the other character has not gotten the message the first time.
Cross your arms over your chest or place hands on your hips (loosely--not exaggerated) when the other character refutes your idea or is not performing a task quickly enough.
Speak authoritatively. One way to do this is to not allow your tone to slide up at the end of a declarative sentence (the way the pitch would sound if you were asking a question)--unless, of course, you are using a sarcastic tone.
Appear unconcerned with the other character's opinion. Your character's opinion is what matters most to him or her. On stage or screen this can be portrayed by turning your back on the other and walking away, or by performing tasks as they start to speak.