He holds her hand fiercely, full of regret, knowing he will never have the chance to tell her how much he loves her, when he decides to take the chance and yanks the wires right out of the bomb.
She feels his hands wrap around her as the pain seers through her chest. Rick rises after tackling Kate to the ground, only to find blood covering her gloved hand. She focuses only on his face, barely feeling the hands gently lifting her as he tells her to stay with him, and that he loves her.
Rick doesn't know what to do with his hands when she tells him she doesn't remember being shot. When she tells him "Some things are better left unremembered", his hands lay in his lap, like dead useless things, while he yearns to reach out, hold her hand and tell her again. When she tells him she will call him, he keeps his hands by his side, walks out the door, and waits.
When Castle slides the note to her, she makes sure with just a look and gentle squeeze of his hand that they would all make it through this some how. And then when Kate, not detective Beckett, extends her hand into his shirt collar and lovingly (that's got to be love he sees in her eyes) says "You're alright", hope for the future, makes it's way back into his heart.
"Hold my hand," says Kate. As Rick cautiously takes her hand. They both pretend that holding hands and working so hard to get free and away from the tiger is all business , but secretly they still have the hope for something more.
Between, almost not being able to let go of Kate's hand over something as silly as a dog, and grabbing Kate's hand to drag her through the murky water of the Hudson to safety, Rick's confidence grows steady that the time is coming, when holding hands will be just part of dreams being brought to life.
Rick's dreams come crashing down around him when he stands mutely outside the box, watching Kate move her hand toward the face of the suspect, pointing at him, and telling him there was no way he could forget the bombing, and that she remembers every single second of her shooting.
The time he remembers most, however, is when Kate walked through his door, sliding her hands over his face, kissing him and apologizing for all the hurt she's caused him. She always remembers the way Rick reacted as the thunder claps and she brings her hand up toward his lips, allowing her fingers to just glance over them in a final plea for forgiveness and understanding.
As they stand before each other now, hands joined, vowing to love each other always, they remember how the simple act of holding hands continues to move them forward in this life they're building together.