I've found that pair programming for a bit can help me get past that initial resistance. It doesn't have to be anything special or official - just a shared screen with a friend over skype can be enough to get going.
I guess I'm a little old school, but the first thing I do is build some datasets. Either from the legacy app or via requirements if it's a greenfield. Seems boring but for me it's simple work and it gets the creative juices flowing. Then it becomes a problem / challenge which is of course interesting.
It doesn't matter whether you are 2, 3 or 4 people. And it also doesn't matter whether you meet with a co-founder or a customer. It helps to meet in RL, but sometimes a plain phone call can be sufficient, too.