basically, your boy's right hand man....
-of course, the bachelor party and gather all the funds for it and planning the location
-if he randomly needs a drink and talk about his "transition" then be there for a nukka
-organize gatherings i.e. tuxedo/suit fitting, etc.
-and coordinate and handle any small shyt that your boy needs
However, when I got married, my best man had to only coordinate the bachelor party and that was it. Ain't need to be catered to like a Groomzilla and shyt.