Many people suppose that acting for television is easy, that you just do what you would do anyway, if it were real life.
But the point is that it isn't real life. So how do you make people believe that it is?
Three classic virtues are to maintain believability, listen to the other actors and share attention.
Another more subtle point relates to timing. Never force an emotional response forward, your job is just to allow it to happen. Likewise, never deliver a line before it has to be delivered. If you wait it will be easy to recognise the right moment.