Approach coming out as a casual or serious conversation?

Hmm, I'd suggest don't try to wrap the message in too many layers of stories/metaphors/etc before getting to the actual punch line... for one, your friend will probably get dizzy trying to follow you along or might even be in a state of frustration when you get to the important part - the other caveat is you might get sidetracked a dozen times and lose your confidence to actually say what you wanted to say. Maybe try something like "Hey it's been a while since we had a good talk, and I'm really excited to tell you about my trip and show you all these pictures - but before we get into that, there's something else I'd like to share with you because you're my best friend and I want you to know...." - then whatever formula is appropriate to your situation at the moment, whether you want to use "I'm gay" or simply "I think I like guys" or "I'm going through some questioning about my sexuality and I've come to the conclusion that I like guys more than girls"...
Best of luck with your announcement .

 

I completely agree with this! You don't want to get sidetracked or lose confidence in yourself! Go by the approach stated above! good luck!