Just as stage actors must project their voice so that they can be heard easily throughout the theater space, actors working in front of a camera must be careful to allow the microphone picking up their voices to do the work. No extra "projection" is necessary. In fact, this projection could actually distort the sound of the voice. Speaking for the camera is like talking to someone sitting next to you. Allow your voice to flow naturally, but make sure to speak clearly. Even up close, speaking too quickly or mumbling can make what you are saying indecipherable to the listener.
Be clear about the camera angle and find out from the director what is actually "in frame" for each shot that you are in. If you are sitting at a table in a scene, for example, know whether your face, torso or whole body will be seen in the camera's frame. Sometimes, you'll actually be "off camera" and only heard while the camera is focused on the person in the scene with you. Knowing which parts of your body are "in frame" for each shot will help you know how to use your body. If the shot is a closeup of your face, be careful not to overwork your facial expressions. This usually appears as overacting and can be distracting to the viewer.
Understanding what the scene is about and what your character wants from the other character in the scene helps make your acting work on camera compelling and interesting to watch. Know your lines cold, so that you can focus your attention on what you are saying and doing as the character, rather than trying to remember the next thing you are supposed to say. Since acting on film is recorded and not a live performance, you may have a few opportunities or "takes" to attempt the scene. Use these takes to offer the director some very different approaches to the scene, so that he will have a variety of possible interpretations to choose from when editing the scene.
Technically speaking, you need to "hit your marks" in each and every scene. This means that you must stand where you are directed to stand and walk and sit exactly where you are directed to walk and sit. The frame of a camera only contains a certain visual space, and the director will be relying on you to hit your marks so that you remain in the light and in the frame. You may have an important piece of action to complete in a scene -- for example, shooting a gun or kissing someone. Know exactly when and how this event is supposed to happen, as there may be other technical aspects of the scene (such as sound) that need to coordinate with your action.