M*A*S*H 4X01 (Welcome to Korea) is prolly the best character relationship building episode.
We get to know BJ, we get to see Hawk missing Trap and hurting, we get to see (what I interpret as) the boys flirting with one another, we get to see BJ getting all of the experiences of Korea in mere hours, and we get to see Hawk comforting him, holding his head while he gets sick saying (I know, and I’m sorry. I know it’s not my fault but I’m sorry. Sorry that good people with families like you are here, sorry that you have to see this, sorry that all of this is happening) and we get to see a connection, a friendship between a man who’s scared and confused and in need of someone to hold his hand and a man who’s angry and lonely a hurt and in need of someone to fix that.
Frank never stood a chance with BJ. From the moment he and Hawk brought Radar into that officer’s club and bounced off of each other so beautifully BJ was hooked, you could see it. And from the moment Hawk took his hand and helped him through that field, BJ’s loyalty would only ever be to Hawkeye.