Not everyone believes in writing out a character history. I think if it works for someone, fantastic. I think any research one wants to do to understand something about the character is great. Especially for externals. And, I say use whatever you can to create a brilliant performance.
But at the end of the day, the writer did all that for you. Your job is to play what's on that page.
Sidenote: one of the greatest advantages that comes from this freedom? Cold readings. Of course, what happens before the scene and after the scene can affect choices made for performance. But if you don't have that information just yet, you can still play the scene.