Another recent decision of the Supreme Court addresses two key points. The first is the need to have agreed contract terms, before work starts under a contract and the second that, where a letter of intent is signed and work starts due to it, that a contract must be finalized as soon as possible after that. In this case, the parties entered into a contract formed by a letter of intent. When the letter of intent expired the work continued on, before the terms of the detailed written contract had been finalized.
I completely understand the smoke_and_mirrors dance producers have to perform to bring all sides to the middle, but when it comes to key talent attachments, if it's not verifiable, it doesn't exist. Bottom line, ditch the letters and let us pick up the phone and verify Mr. Jackman.