Not interchangeable. There are some cases where swapping them would give very odd results: "It was raining yesterday", not "It were raining yesterday". "We were tired", not "We was tired". It's only in the subjunctive -- conditionals, counterfactuals, desires and so on -- that was and were are sometimes interchangeable. "Was" is possibly always safe nowadays, "Were" is perhaps better in some cases (you'll often hear "If I was you..." but I think "If I were you..." still sounds better, and some editors might insist on it).