Show. Don't tell.

"Have a secret."
This was the easiest piece of advice I have received in an actor's studio. These words (highly paraphrased) came from the mouth of perhaps one of the finest actors of our generation, Meryl Streep. She was asked about her process as an actress. It was later shared with me by an excellent acting coach. These words changed my approach to character instantly and broke that wall between myself and the role I performed.
Consider this.
We ALL have secrets. Every single one of us hides something about ourselves deep down. The guarding of this little truth informs so many of our decisions. They become who we are outwardly in many ways. They govern who we interact with and how we interact with them. They inform every relationship we have.
If you want a strong base to develop a character from as an actor, start by giving them a secret. It will help you understand your relationship with the other actor in the scene. It will give context to every line you say. It will help stir that inner-life that shows up so brilliantly on camera. Best of all, if you lose your way in a scene, you can always return to that little secret your character has and spring back to life.
What can that secret be?
Well, pretty much whatever feels right to you and your character. It should be something that is in line with the context of the story, certainly, but ultimately its only importance is that it gets you acting.
For an example, I will return to Meryl.
In the film "Kramer Vs. Kramer" She plays Mrs. Kramer, a woman in the middle of a divorce with her husband, played by another luminary, Dustin Hoffman. She says in her interview that she created Mrs. Kramer's inner life with a simple secret, one that informed every scene she shared with Dustin:
"She never really loved him."
She never told anyone her character's secret during the entire production of the film. It was for her alone, and it was used to stunning effect.
Meryl won the Academy Award for Best Actress in a Supporting Role in Kramer Vs. Kramer in 1979.
In general, I typically refrain from giving acting advice. It is such a personal process, and honestly my own approach is in perpetual flux. I take what works, I disregard what doesn't, and I stumble on.
However, I confess that I do suffer from a healthy dose of hero worship.
When Meryl talks, I listen with every nerve that accepts raw data.
The changes to my work through this simple piece of advice were subtle yet profound. Give it a try and see what happens.