To react is to accept another's authority to change you.
Indifference is the the worst thing in improv. Indifference to an offer is the very essence of blocking it. Ignoring or not caring about your partners is exactly what will make others not want to play with you in the future.
Improv is all about reaction, not about action. And real reaction invokes change. This is the reason why playing (very) drunk people, (very) young children or otherwise characters that don't react but live more in their own world is so very difficult to pull of in improv.
Accept anything to change your character. This doesn't mean you should change your mind all the time (a strict vegetarian should never accept that sausage). It can also mean you change your emotion, change your status, change your energy, change the speed/volume/pitch of speaking, etc...
If you're serving your partner, you accept their authority to change you.