When acting for the camera keep it real.
Don’t over-emote verbally or physically. Keep it simple, straightforward and real.
You’re not playing to a live theater audience but simply to the actor opposite you; therefore you under- play as opposed to over-playing the role. You’re simply having a conversation, argument, whatever, with the other actor – as you would in real life – so underplaying for the camera is far more effective than “acting” and pandering to the audience.
The camera can either be your best friend or your worst enemy. The camera demands subtlety and does not respond well to over-acting, either verbally or physically.
Don’t pander on-screen to the audience, keep it simple, passionate and real. If you fully inhabit the role in a scene then the audience will be able to put an accurate spin on what you are thinking and feeling.
In other words, don’t give it all away. A sense of mystery when it comes to the character will always keep the viewer interested and engaged.