It may seem obvious, but know your lines. A lot of people worked really hard to get those words right, so honor them and deliver them as they are written. If something really doesn’t roll off your tongue, talk to the Director, don’t rewrite the script to accommodate yourself.
On the flip side, don’t over practice your lines; give some room for discovery and for taking direction. The brain is a funky thing, if you practice your dialogue too much in one way, it becomes ingrained and the camera captures a robotic performance.
There’s a lot of talk about overlapping dialogue. Here’s the skinny: It’s okay to overlap dialogue if it happens naturally, like in a fight, and if you are in a wide or a two shot (when both Actors are on camera at the same time). If you are in a single, Editors like to have the dialogue clean, so wait for the other Actor to finish the line.
Editors can overlap dialogue if they need to. Also, make sure that you don’t step on any lines. Sometimes newer Actors are so prepared that the words come out so fast that the other character hasn’t finished their line. The best way to avoid this is to remember to listen.
And last but not least, try to avoid grunts and groans. Unless “Uh” or “Ah” is in the script, leave it out.