You may have an actor of a certain budget who has no roles lined up currently, but is a pretty safe bet he will get some lined up eventually, and so he's a decent risk for a loan.
This is private lending and is a completely different world than a home loan that is resold. Depending on the dollar amount, the lender will have their own appraisers, etc taking careful look at the collateral (which might be the castle you're buying, or that and more, or something else entirely, like royalties due, etc).
They will then structure it so that it's a heads they win, tails they don't lose - only lending as much as they're sure they'll be able to get back out (up to and including having alternate buyers lined up to purchase the property if it gets foreclosed, etc).