Jesus' instructions in Luke 17:3-4 are really quite simple, and are no exception to the general rule that repentance involves a change in behavior:
Be careful. If your brother sins against you, rebuke him. If he repents, forgive him. If he sins against you seven times in the day, and seven times returns, saying, ‘I repent (metanoō),’ you shall forgive him.
Repeated sins are to be met with repeated forgiveness. When one who has sinned against me many times comes to me again, professing he has repented, I am to freely forgive him again. How do I know that he didn't fully repent last time he came to me? Because he committed the same sin against me again. But what is to be my expectation when he comes to me saying he has repented, again. I am to expect that he will change his behavior this time. Or, at least, I am not supposed to permit my bad past experience with him poison my present expectation, so that I will withhold forgiveness in the expectation that he will only do it again.
So the main emphasis of this passage is the command to forgive, even repeatedly, when faced with professed repentance--just as God forgives us repeatedly. But the assumption that true repentance should lead to changed behavior certainly underlies the passage.