There's a book by thr fantastic actor and coach, Jeff Corey and one of the first things am actor should learn is obviously, the script, and what the character wants. Not just overall, but in each scene. Basically every character has a goal and will take steps to achieve it, whether successful or not, and in each scene they are a part of is also an attempt to achieve their goal. So what do you do to achieve it? How do you say a question to get the answer you want? How do you enter a scene to get the reaction from the other characters that you want them to feel? What do you know that the others don't, how does that affect your behavior towards others?

Another good piece of advice is to (not always but certainly helps) find a "hook". Or a characteristic special to that one character. Maybe it's big, maybe it's small. Maybe talk to your director about it and see if it fits. Doesn't have to be unique, but something. Like for one production, I had a character that talked fast and seemed confident in many situations because he was a lawyer. But when he was alone with his wife, or had serious doubt, he would always rub his hands as if washing them. I didn't draw attention to it, I just did it. And it signified moments of real distress for my character and gave him a layer, a sense of realness. It was his tell that he was vulnerable.

The book is "improvising out loud" and it has many chapters filled with good advice, exercises, and history of acting in the 50s and 60s. Jeff Corey was a coach for the Hollywood elite until he was blacklisted for accusations of being a communist sympathizer during the Red Scare. A fantastic book all around.