[disclaimer: this is just my take from far away and over the internet without a lot of direct context]
1. Calling the graphic designer yourself to find out about payment is basic due diligence. No one in business should take offense at it, in case you're worried. If someone does, that's not good.
2. If they're bringing you in as an equal equity holder, vagueness about funding isn't really a good basis for a partnership.
3. Tying into a movie release isn't a solid marketing plan for a startup. Sure it's nice to get a boost from lucky timing, but there needs to be a Plan A.
4. No buy sell agreement combined with pressure to commit is absurd: commit to what?
5. If the software is the critical bit of their business, not being into the software is not being into the critical bits of their business. Work with people who will take out the garbage and jump up and down on top of the dumpster if it won't fit when that's what needs to be done.
Questions:
Do they have a track record that you're not mentioning?
What are they doing day in and out to add value?
How will they be adding value if you come on board during the time you are working on the code?
Why no buy-sell agreement?
Why no business and marketing plan as formal documents?
What do you think will happen if you decline their offer?