In most commercial disputes a small number of documents determine the outcome, and they are identifiable early.
The contract and its history. Executed version, prior drafts, redlines, and the covering emails. Negotiation history matters where ambiguity is claimed, and the drafts frequently show which side proposed the disputed language.
Contemporaneous internal communications. What people said when they had no reason to be careful. Chat and messaging platforms have largely replaced email for these, which is why collecting only email misses the case.
Financial records. Invoices, payment history, account statements, and the accounting entries that show how a transaction was actually treated. Accounting treatment is often the most candid statement of what the parties believed.
Approval records. Board and management approvals, delegations, and the memoranda supporting them.
Third-party documents. The counterparty’s communications with its own customers, lenders and advisers, obtainable by subpoena, are frequently more useful than anything either party produced.
The absence of a document is evidence too. A decision of significance with no contemporaneous record supports the inference that the stated reason was formed later.
Practical instruction. Build the chronology from documents before interviewing anyone. Memory conforms to the story a witness has already told; documents do not.