@jeffvmaharIs it true that a Beastmaster’s animal companion will just sit there being attacked unless I use my action to command it?
nope – dm's call, but it's still a creature, not a robot. would protect itself IMO

— Mike Mearls (@mikemearls) June 14, 2015

@jeffvmaharOk great. Now if I use my action to give it an attack command, will it continue to attack or do I need to spend another action?
I'd run it like this – command get it to do exactly what you want, DM can make judgment calls keeping in mind it's an animal

— Mike Mearls (@mikemearls) June 14, 2015