I'd rather translate "to take for granted' as "prendre pour acquis" but I'm not sure I understand very well your sentence. What does "to take a BEHAVIOR for granted" mean?

"prendre pour acquis" or "aller de soi" are good, "prendre comme une évidence" is not. It's hard to corner this phrase in French. We do say similar stuff, but maybe differently. You could say for example "son attitude était normale pour elle". (his behavior was normal for her) or "elle a présumé qu'il allait réagir normalement". (she assumed he would react normally) If I translate: "she took is behavior for granted, she didn't even consider the possibility of his behavior not being polite. From his appearance she concluded automatically that he is going to have good manners." I'd say : "elle avait pris pour acquis la façon dont il allait réagir. Elle n'avait même pas considéré l'option qu'il puisse être impoli. À partir de l'apparence qu'il projetait, elle a conclu automatiquement qu'il devait avoir de bonnes manières."

I agree with Matt, to take a behavior for granted, not sure what that means :) prendre pour acquis seems good to me :)