Credit: ABC Photo: Alexis Helps Castle in 3

In the hustle and bustle of any given Castle episode, it can be easy to miss what a seriously good actor Nathan Fillion really is. The genius of Fillion is that he sells all of his conversations on the show, regardless of how unlikely the scenario. Let’s take a look at three moments from last night, as Castle exhausted himself defending a friend who soon turned foe.

1. The Poignant Moment

The Scene: In which you confide to a trusted associate that your entire identity is wrapped up in something a 16-year-old literary editor once told you.   

The Tone: Desperate, wistful

Beckett had trouble understanding why Castle wouldn’t just let her put his seemingly guilty pal Damian behind bars and swallow the key. Isn’t that what friends do? Castle’s explanation was both confounding and touching. Damian was the literary editor at Castle’s boarding school and the first person to recognize his writing talent. “Without Damian Westlake, I’m a lawyer, I’m a grafter, I’m a rodeo clown,” Castle said. “But I am not a writer. Without him, I’m not me.”

Of course, Castle would still be a best-selling author even with Damian in prison, but with great poignancy Fillion showed how Castle has permanently linked his friend to the only known cure for homesickness: writing.

2. The Aha! Moment

The Scene: In which you rouse a retired detective from his crime-solving slumber to show him your yearbook.

The Tone: Determined, energized

Castle invited Detective Ray Salazer out to coffee to make a joke about Manhattan retirement living, and to let him know he and his daughter solved in 10 minutes a case he couldn’t crack in over two decades. Damian’s father, Phillip, was killed in Puerto Rico many moons ago, and Castle was determined to clear his friend’s name in that case. Fillion is the perfect "Guy Who’s Onto Something."

"Maybe it was this crazy sociopath Michael Rutherford who did it! Maybe he flew to Puerto Rico to kill Phillip and get rich! Maybe Damian had nothing to do with it!" Mmm...hold off on that last part. This brings us to our third moment.  

3. The Quietly Enraged Moment

The Scene: In which you tell your friend that he’s not going to Barcelona because he is a lying bastard.

The Tone: Calm and resigned, masking a deep anger

Well, Damian didn’t kill his wife. But in his and Castle’s final exchange, you can tell right away from the change in Fillion’s expression that the old friend isn’t so innocent. Damian probably shouldn’t have given Rutherford so many of his personal belongings in exchange for killing his dad. Or a hand-drawn map of the house. Kids! “I told them you’d be a gentleman,” Castle said, as the police rang the bell. A little over-the-top? Maybe. But it's TV, and Fillion kills it.

Credit: ABC Photo: Sneak Peek Clip of Castle Season 3, Episode 15: "The Final Nail"