If he didn't already have "earnest & sincere" down pat, I might have been surprised. However, think of his scenes with Kaylee, and you can see the caring protector vibe at work.
Part of what makes Castle's relationship with his daughter work (I've really got to start learning names besides Castle & Beckett) is the maturity of the daughter. Castle could not have been as good a father to a rebellious, angry daughter. The fact that she is wise for her years makes her a stabilizing influence on him. And yet, you can see that when it comes to his daughter, Castle knows when to take life seriously and put her first.
Her sweet maturity brings out the best in him. Very well-designed character interaction. Love it.