Archer is going to be the town drunk in Capers. He'll initially reject this role, but when this means the townspeople try to drive him away from the railroad baroness, Miss Tohsaka, he'll grudgingly start accepting it.

Which means, since he doesn't have the greatest tolerance for alcohol, that he'll be constantly slightly tipsy and desperately trying to discreetly dispose of drinks bought for him by the locals.

That said, he'll manage to shake off the influence once he realizes he's being stalked by an actual user of projectile weapons (way to be preempted, Archer). However, he still won't take this guy very seriously, since guns are not usually a big threat to Servants. If anyone else wants to get involved in the final showdown and wind up hurt because of Archer's underestimation of his enemy, that might be cool.