Watch this video up until about 5:45 or so (I have it set to only be a minute). David Tennant's expression right there, when they start joking about "is Davros the one that looks like a prawn?"
That expression says, "I really just want to keep talking about my fandom, but I recognize that you don't know/don't care about how serious I am about my fandom, and if I keep going, you will be bored, and consider me a weirdo. So I'm going to back off and joke about prawns with you, and look normal and like I'm not taking this seriously even though I really just want to keep going about my show."
I have LIVED that expression.
That expression says, "I really just want to keep talking about my fandom, but I recognize that you don't know/don't care about how serious I am about my fandom, and if I keep going, you will be bored, and consider me a weirdo. So I'm going to back off and joke about prawns with you, and look normal and like I'm not taking this seriously even though I really just want to keep going about my show."
I have LIVED that expression.